In this cohort study, dynamic trunk extensor performance was studied as a predictor of permanent work disability due to back disorders. As part of the comprehensive Mini-Finland Health Survey in 1978-80, the back muscle performance of 535 persons (267 men, 268 women) was measured using standardized repetitive arch-up and sit-up tests. At baseline, the participants were between 30 and 64 years of age. Retirements were followed for 12 years on average. During the follow-up, 56 subjects developed permanent work disability; 15 of these cases were back-related. Good dynamic trunk extensor performance was predictive of a decreased incidence of work disability due to chronic back disorders but not work disability due to other diseases. The risk of back-related work disability in the three highest quartiles in relation to the lowest quartile of dynamic trunk extension capacity was 0.28 (95% confidence interval, 0.09-0.94). Our study suggests that good dynamic trunk extension performance may protect against back-related permanent work disability.