Chronic insomnia is common in the general population. Its effect on functioning and disability is usually attributed to an underlying condition, so the diagnosis of insomnia does not qualify for award of a disability pension in the United States or Europe. The aim of this study was to investigate whether insomnia, defined according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, contributed to long-term work disability. Using a historical cohort design, the authors gathered baseline data from a population-based Norwegian health study of 37,308 working-age people not claiming a disability pension through 1995-1997. The outcome was subsequent award of a disability pension (18-48 months after the health screening) as registered by the National Insurance Administration. Insomnia was a strong predictor of subsequent permanent work disability (adjusted odds ratio=3.90, 95% confidence interval: 3.20, 4.76). Sociodemographic and shift-work characteristics had little confounding effect (adjusted odds ratio=3.69, 95% confidence interval: 3.00, 4.53), and this association remained significant after adjustment for psychiatric and physical morbidity and for health-related behaviors (adjusted odds ratio=1.75, 95% confidence interval: 1.40, 2.20). This study suggests that insomnia should receive increased attention as a robust predictor of subsequent work disability.