OBJECTIVE: To update a previous systematic review on the effectiveness of worksite-based weight loss programs. DATA SOURCE: The following databases were searched: Medline, PsychlNFO, Embase, The Cochrane Library, and LexisNexis. STUDY INCLUSION AND EXCLUSION: Studies were limited to those published in English from 1995 to 2006 to which the following inclusion criteria were applied: (1) worksite intervention, (2) body weight assessed before and after intervention, and (3) study duration of at least 8 weeks. DATA EXTRACTION: Data were extracted on the following: study design; funding source; purpose of evaluation; participant and worksite characteristics; type, intensity and duration of intervention; primary and secondary outcomes; and methodological quality. DATA SYNTHESIS: Heterogeneity of study designs precluded quantitative data synthesis. Results. We identified 11 randomized controlled trials, most of which focused on education and counseling to improve diet and increase physical activity. Follow-up ranged from 2 to 18 months, with 56% to 100% of subjects completing the studies. The overall methodological quality of the studies was poor. Intervention groups lost significantly more weight than controls, with the mean difference in weight loss ranging from -0.2 to -6.4 kg. CONCLUSION: Worksite-based weight loss programs can result in modest short improvements in body weight; however, long-term data on health and economic outcomes are lacking. So What? There is a need for rigorous controlled studies of worksite-based interventions that integrate educational, behavioral, environmental, and economic supports.