Objective: To compare the effect of function-centered treatment (FCT) and pain-centered treatment (PCT) on the number of work days. permanent disability, and the unemployment rate. Design: Randomized controlled trial. Setting: Inpatient rehabilitation center. Participants: Patients (N= 174; 79% male;, mean age, 42y) with previous sick leave of 6 weeks or more. Interventions: FCT (4h/d for 3wk) emphasized activity despite pain by using work simulation, strength, endurance, and cardiovascular training. PCT (2.5h/d for 3wk) emphasized pain reduction and included passive and active mobilization, stretching, strength training, and a 4-hour mini back school with education and exercise. Analysis was by intention to treat. Main Outcome Measures: Work days, return to work, rate of patients receiving financial compensation for permanent disability. and unemployment rate. Effect sizes (Cohen d) were defined as small (0.2-0.5), moderate (0.5-0.8), and large (> 0.8). Results: After 1 year, the FCT group had significantly more work days (mean. 118; median, 39.5; interquartile range [IQR]. 0-198) than the PCT group (mean, 74; median, 0; IQR 0- 160: Mann-Whitney U test, P=.011). The odds ratio of returning to work in the FCT group relative to the PCT group Was 2.1 (95% confidence interval, 1.1 - 3.9). The differences in unemployment rates and in the numbers of patients receiving, compensation for permanent disability were not significant. Conclusions: FCT is more effective than PCT for increasing work days.