A strategy is needed on how to treat the growing number of obese children with the limited resources available. We compared the long-term (24 months) effectiveness of therapist-led groups (TLG) v. self-help groups (SHG) for parents on changes in children's adiposity and dietary intake. The study included ninety-nine children (forty-eight girls) who were referred to obesity treatment (7–12 years, BMI z-scores ≥ 2, attendance of at least one parent). Parents (ninety-one mothers, fifty-four fathers) were randomised to TLG aimed at increasing parents' competence to accomplish lifestyle changes (n 47), or SHG (n 52), both with fifteen sessions. All children participated in children's groups, and all families attended individual counselling by a clinical dietitian and physiotherapist. Percentage of body fat (BF) was measured by dual-energy X-ray absorptiometry, BMI z-score was calculated by international reference values and dietary intake was calculated from 4 d estimated food records at baseline and after 6 and 24 months. No significant between-group differences were detected in the children's changes in adiposity or dietary intake after 6 and 24 months. BF, BMI z-scores and energy intake were significantly decreased after 6 months (P< 0·05) in both intervention groups, and this persisted throughout 24 months without compromising the diet macronutrient composition. In conclusion, the TLG and SHG intervention groups appear to be equally effective in improving long-term adiposity and dietary intake in obese children. Further research should be performed to clarify whether the SHG should be preferred to parental group treatment for similar children with obesity.