Objective: To evaluate the cost-effectiveness and cost-utility of a return to work (RTW) program for workers on sick-leave due to low back pain (LBP), comparing a workplace intervention implemented between 2 to 8 weeks of sick-leave with usual care, and a clinical intervention after 8 weeks of sick-leave with usual care. Design: Economic evaluation alongside a randomised controlled trial (RCT). Study population: Workers sick-listed for a period of 2 to 6 weeks due to LBP. Interventions: 1. workplace assessment, work modifications and case management). 2. physiotherapy based on operant behavioural principles. 3. usual care: provided by an occupational physician. Outcomes: The primary outcome was return to work (RTW). Other outcomes were pain intensity, functional status, quality of life and general health. The economic evaluation was conducted from a societal perspective. Outcomes were assessed at baseline (after 2-6 weeks on sick-leave), and 12 weeks, 26 weeks, and 52 weeks after the first day of sick-leave. Results: The workplace intervention group returned to work 30.0 days (95% CI=[3.1, 51.3]) earlier on average than the usual care group at slightly higher direct costs (ratio of 1 day: €19). Workers in the clinical intervention group that had received usual care in the first 8 weeks returned to work 21.3 days (95% CI= [-74.1, 29.2]) later on average. The group that had received the workplace intervention in the first 8 weeks and the clinical intervention after 8 weeks returned to work 50.9 days (95% CI=[-89.4, -2.7]) later on average. A workplace intervention was more effective than usual care in RTW at slightly higher costs and was equally effective as usual care at equal costs on other outcomes. A clinical intervention was less effective than usual care and associated with higher costs. Conclusion: The workplace intervention results in a safe and faster RTW than usual care at reasonable costs for workers on sick-leave for two to six weeks due to LBP. © 2006 Springer Science+Business Media, LLC.