OBJECTIVE: The objective of this study was to determine whether a lifestyle intervention with individual counselling was cost-effective for reducing body weight compared with usual care from a company perspective. PARTICIPANTS: Overweight employees were recruited and randomly assigned to the intervention groups, either phone or Internet, or the control group. METHODS: The intervention was based on a cognitive behavioural approach and addressed physical activity and diet. Self-reported body weight was collected at baseline and 12 months follow-up. Intervention costs and costs of sick leave days based on gross and net lost productivity days (GLPDs/NLPDs) obtained from the participating companies were calculated. Missing data were imputed using multiple imputation techniques. Uncertainty surrounding the differences in costs and the incremental cost-effectiveness ratios (ICER) was estimated by bootstrapping techniques, and presented on cost-effectiveness planes and cost-effectiveness acceptability curves. RESULTS: No statistically significant differences in total costs were found between the intervention groups and control group, though mean total costs in both intervention groups tended to be higher than those in the control group. The ICER of the Internet group compared with the control group was €59 per kilogram of weight loss based on GLPD costs. The probability of cost effectiveness of the Internet intervention was 45% at a willingness-to-pay of €0 per extra kg weight loss and 75% at a willingness-to-pay of €1500 per extra kg body weight loss. Comparable results were found for the phone intervention. CONCLUSIONS: The intervention was not cost effective in comparison with usual care from the company perspective. Due to the large amount of missing data, it is not possible to draw firm conclusions.