Little is known about predictors of duration of work disability (DOD). This cohort study of 433 workers' compensation claimants estimated DOD for job, injury, and demographic factors during consecutive disability phases using Cox regression analysis. DOD was calculated from administrative records. Results show that DOD increases with the time spent bending and lifting or pushing or pulling heavy objects at work, but it is unrelated to sitting, standing, or vibration. Younger age, longer pre-injury employment, less severe injuries, and a previous back injury predicted shorter disability, the latter factor only during the subacute/chronic disability phases. The effect of injury severity decayed over time. This study demonstrates the usefulness of a phase-specific analysis and shows that physical job and injury factors have a significant and time-varying impact on DOD.