Pre-Existing Conditions & Workers’ Comp: Employee’s Guide

A pre-existing condition does not stop you from getting California workers’ comp. If your job caused a new injury, or aggravated, sped up or “lit up” a condition you already had, you can generally get medical treatment and temporary disability benefits for the work-related injury. Where your old condition matters most is permanent disability, which can be reduced through a process called apportionment.

Why a pre-existing condition doesn’t bar your claim

Workers’ comp doesn’t require you to be in perfect health when you’re hired. It covers injuries that arise out of and occur in the course of employment (often shortened to AOE/COE). California workers’ comp law has long treated a work-related aggravation of an existing condition as an injury in its own right.

Common examples include:

• A back with mild degenerative disc disease that becomes disabling after repeated lifting
• A knee with old arthritis that tears after a fall on the job
• An old shoulder surgery site re-injured by overhead work
• Carpal tunnel symptoms that worsen with constant keyboard or tool use

Insurers often argue that your symptoms are “just your old condition.” The real question is whether work caused, contributed to or worsened your condition, and what a doctor says about it.

Temporary flare-up or permanent worsening

Doctors usually separate two situations:

• Temporary aggravation. Work made your condition flare, but you are expected to return to your prior baseline. You can generally receive treatment and temporary disability while you recover, but there may be little or no permanent disability.
• Permanent aggravation. Work left you permanently worse than before. That can support a permanent disability rating, subject to apportionment.

Temporary disability generally pays two-thirds of your average weekly wage. For injuries in 2026, it is at least $264.61 and at most $1,764.11 a week, according to the Division of Workers’ Compensation. See our guide to temporary disability benefits.

How apportionment works

Apportionment divides your permanent disability between the work injury and other causes. Two statutes control it:

• Labor Code §4663. Apportionment of permanent disability “shall be based on causation.” A doctor reporting on permanent disability must state what approximate percentage was caused by the work injury and what percentage was caused by other factors before and after it, including prior industrial injuries. If the doctor can’t do that, the report must explain why.
• Labor Code §4664. Your employer is liable only for the percentage of permanent disability directly caused by the work injury. If you received a prior permanent disability award, the law conclusively presumes that the earlier disability still exists. Total awards for one region of the body generally can’t exceed 100 percent over your lifetime.

The doctor making this call is usually your primary treating physician, a panel Qualified Medical Evaluator (QME) or an Agreed Medical Evaluator (AME). Our guide to QMEs and AMEs explains the difference, and our Labor Code §4663 explainer covers the statute in more depth.

What a valid apportionment opinion needs

A percentage alone isn’t enough. A 2005 en banc decision of the Workers’ Compensation Appeals Board held that an apportionment opinion must be substantial medical evidence. In practice that means the doctor must explain how and why the other factor, such as degeneration or a prior injury, is causing part of your current permanent disability, based on your history, exam and records.

Apportionment opinions are often weakest when:

• The doctor relies only on an X-ray or MRI showing “degeneration” without linking it to your actual disability
• You were working full duty with no restrictions or treatment before the work injury
• The report assumes facts about your history that aren’t true
• The doctor confuses the cause of the injury with the cause of the disability

If an opinion has these problems, your attorney can request a supplemental report, depose the doctor or object to the report.

A worked example

Here is a hypothetical. Rosa, a warehouse worker injured in 2025 who earns more than $435 a week, hurts her lower back lifting boxes. An MRI shows degenerative disc disease, but she had no prior treatment or work restrictions. The QME rates her back at 20 percent permanent disability and apportions 25 percent of it to degeneration.

The apportionment cost Rosa $7,250. If her attorney shows the 25 percent figure isn’t supported, she may recover the difference. The apportionment doesn’t reduce the treatment and temporary disability she receives for the work injury. Run your own numbers with our workers’ comp benefits calculator.

Common insurer arguments and how workers respond

• “You were already hurt.” A work-related aggravation is still a work injury. Point to your work history, full-duty status and lack of recent treatment before the incident.
• “It’s just degeneration.” Degeneration on imaging is common and often causes no symptoms. The question is what caused your disability now.
• “You didn’t tell us about your old injury.” Disclose prior injuries when asked. Hiding them can damage your credibility and, in serious cases, raise fraud concerns.
• “Your claim is denied because of your history.” A denial can be challenged. See our guide to a denied workers’ comp claim.

Steps to protect your claim

1. Report the injury quickly. Give written notice within 30 days (Labor Code §5400) and complete a DWC-1 claim form.
2. Describe what changed. Tell every doctor what you could do before the injury and what you can’t do now.
3. Give a complete, accurate history. Include prior injuries, treatment and any earlier claims.
4. Gather proof of your baseline. Job descriptions, attendance records, prior medical records and coworker statements can show you were working normally.
5. Watch the deadline. Proceedings generally must start within one year of the injury or of the last benefit payment or treatment furnished (Labor Code §5405).

Frequently asked questions

Can I get workers’ comp if I had back problems before?

Yes, if work caused a new injury or aggravated your condition. Your permanent disability may be apportioned, but you can generally still receive treatment and temporary disability for the work injury.

Does apportionment reduce my medical care?

Apportionment applies to permanent disability. It generally does not cut the medical treatment you need for the work injury.

Can my employer deny my claim because it knew about my condition when it hired me?

No. An employer’s knowledge of your condition doesn’t excuse it from covering a new work-related injury or aggravation.

Will an old workers’ comp award reduce my new claim?

It can. If you received a prior permanent disability award, the law presumes that disability still exists (Labor Code §4664(b)), which may reduce what the employer owes for the same body part.

Can I challenge a doctor’s apportionment?

Yes. If the opinion doesn’t explain how and why the other factor contributes to your disability, it may not be substantial evidence, and it can be challenged through a supplemental report, deposition or objection.

Talk to a California workers’ comp lawyer

Employees First Labor Law represents workers exclusively, including injured workers whose claims are being cut because of a prior condition. We offer free virtual consultations to review your medical reports and apportionment. Contact us online or call (310) 853-3461.

This article is general information about California law as of 2026, not legal advice for your situation. Reading it does not create an attorney-client relationship.