Background Weight management constitutes a substantial problem particularly among groups of low socio-economic status. Interventions at work places may be a solution, but high quality worksite interventions documenting prolonged weight loss are lacking. This paper presents results of an intervention aimed to achieve a 12 months weight loss among overweight health care workers.Methods Ninety-eight overweight female health care workers were randomized into an intervention or a reference group. The intervention consisted of diet, physical exercise and cognitive behavioral training during working hours 1 hour/week. The reference group was offered monthly oral presentations. Several anthropometric measures, blood pressure, cardiorespiratory fitness, maximal muscle strength, and musculoskeletal pain were measured before and after the 12-months intervention period. Data were analyzed by intention-to-treat analysis.Results The intervention group significantly reduced body weight by 6.1 kg (p<0.001), BMI by 2.2 (p<0.001) and body fat percentage by 2.8 (p<0.001). There were no statistical reductions in the control group, resulting in significant differences between the two groups over time.Conclusion The intervention generated solid reductions in body weight, BMI and body fat percentage among overweight female health care workers over 12 months. The positive results support the workplace as an efficient arena for weight loss among overweight females.Trial registration: NCT01015716.