Objective: The aim of the study was to compare the efficacy of group treatment stressing a health-promoting lifestyle with routine counseling in the treatment of childhood obesity. Design and subjects: Seventy obese children (weight for height 115-182%) aged 7-9 years were randomized either to routine counseling (two appointments for children) or to family-based group treatment (15 separate sessions for parents and children). These sessions included nutrition education, physical activity education and behavioral therapy. Outcome measures: Children's weights and heights were measured at baseline, after the 6-month intervention and after the 6-month follow-up. The change of weight for height based on Finnish growth charts was used as the primary, and changes in body mass index (BMI) and BMI standard deviation scores (BMI-SDS) as secondary outcome measures. Results: Children attending the group treatment lost more weight for height (6.8%) than children receiving routine counseling (1.8%) (P ¼ 0.001). The difference was significant when the data were analyzed in four groups by the cut-off limits of 0, À5 and À10% for the change in weight for height. The respective decreases in BMI were 0.8 vs 0.0 (P ¼ 0.003) and in BMI-SDS 0.3 vs 0.2 (P ¼ 0.022). The results remained similar in adjusted analyses. Both group and routine programs were feasible with a high, 87-99%, participation rate in sessions and appointments and very low, 3% or less, attrition rate from the programs. Six months after the intervention, beneficial effects were partly lost, but for changes in weight for height and BMI, the differences between the two treatment programs still were significant, and for BMI-SDS, there was a trend. Conclusions: Family-based group treatment that stresses a health-promoting lifestyle and is given separately for parents and children, offers an effective mode of therapy to treat obese school-aged children.