OBJECTIVE: To determine whether ascertainment of childhood obesity by surveillance followed by structured secondary prevention in primary care improved outcomes in overweight or mildly obese children. DESIGN: Randomised controlled trial nested within a baseline cross sectional survey of body mass index (BMI). Randomisation and outcomes measurement, but not participants, were blinded to group assignment. SETTING: 45 family practices (66 general practitioners) in Melbourne, Australia. PARTICIPANTS: 3958 children visiting their general practitioner in May 2005-July 2006 were surveyed for BMI. Of these, 258 children aged 5 years 0 months up to their 10th birthday who were overweight or obese by International Obesity Taskforce criteria were randomised to intervention (n=139) or control (n=119) groups. Children who were very obese (UK BMI z score >or=3.0) were excluded. INTERVENTION: Four standard consultations over 12 weeks targeting change in nutrition, physical activity, and sedentary behaviour, supported by purpose designed family materials. Main outcomes measures Primary measure was BMI at 6 and 12 months after randomisation. Secondary measures were mean activity count/min by 7-day accelerometry, nutrition score from 4-day abbreviated food frequency diary, and child health related quality of life. Differences were adjusted for socioeconomic status, age, sex, and baseline BMI. RESULTS: Of 781 eligible children, 258 (33%) entered the trial; attrition was 3.1% at 6 months and 6.2% at 12 months. Adjusted mean differences (intervention - control) at 6 and 12 months were, for BMI, -0.12 (95% CI -0.40 to 0.15, P=0.4) and -0.11 (-0.45 to 0.22, P=0.5); for physical activity in counts/min, 24 (-4 to 52, P=0.09) and 11 (-26 to 49, P=0.6); and, for nutrition score, 0.2 (-0.03 to 0.4, P=0.1) and 0.1 (-0.1 to 0.4, P=0.2). There was no evidence of harm to the child. Costs to the healthcare system were significantly higher in the intervention arm. CONCLUSIONS: Primary care screening followed by brief counselling did not improve BMI, physical activity, or nutrition in overweight or mildly obese 5-10 year olds, and it would be very costly if universally implemented. These findings are at odds with national policies in countries including the US, UK, and Australia. TRIAL REGISTRATION: ISRCTN 52511065 (www.isrctn.org).