OBJECTIVE: To compare a comprehensive lifestyle intervention for overweight children performed in groups of families with a conventional single-family treatment. Two-year follow-up data on anthropometric and psychological outcome are presented. DESIGN: Overweight and obese children aged 6-12 years with body mass index (BMI) corresponding to >/=27.5 kg/m(2) in adults were randomised to multiple-family (n=48) or single-family intervention (n=49) in a parallel design. Multiple-family intervention comprised an inpatient programme with other families and a multidisciplinary team, follow-up visits in their hometown, weekly physical activity and a family camp. Single-family intervention included counselling by paediatric nurse, paediatric consultant and nutritionist at the hospital and follow-up by a community public health nurse. Primary outcome measures were change in BMI kg/m(2) and BMI SD score after 2 years. RESULTS: BMI increased by 1.29 kg/m(2) in the multiple-family intervention compared with 2.02 kg/m(2) in the single-family intervention (p=0.075). BMI SD score decreased by 0.20 units in the multiple-family group and 0.08 units in the single-family intervention group (p=0.046). A between-group difference of 2.4 cm in waist circumference (p=0.038) was detected. Pooled data from both treatment groups showed a significant decrease in BMI SD score of 0.14 units and a significant decrease in parent-reported and self-reported Strength and Difficulty Questionnaire total score of 1.9 units. CONCLUSIONS: Two-year outcome showed no between-group difference in BMI. A small between-group effect in BMI SD score and waist circumference favouring multiple-family intervention was detected. Pooled data showed an overall improvement in psychological outcome measures and BMI SD score. TRIAL REGISTRATION NUMBER: NCT00872807, http://www.clinicaltrials.gov.