BACKGROUND: Obesity runs in families, and family-based behavioral treatment (FBT) is associated with weight loss in overweight/obese children and their overweight/obese parents. This study was designed to estimate the costs and cost-effectiveness of FBT compared to separate group treatments of the overweight/obese parent and child (PC). METHODS: Fifty overweight/obese 8- to 12-year-old children with overweight/obese parents were randomly assigned to 12 months of either FBT or PC treatment program. Assessment of societal costs (payer plus opportunity costs) were completed based on two assumptions: (1) programs for parent and child were available on separate days (PC-1) or (2) interventions for parent and child were available in the same location at sequential times on the same day (PC-2). Cost-effectiveness was calculated based on societal cost per unit of change using percent overBMI for children and weight for parents. RESULTS: The average societal cost per family was $1,448 for FBT and $2,260 for PC-1 (p < 0.001) and $2,124 for PC-2 (p < 0.001). Child cost-effectiveness for FBT was $209.17/percent overBMI, compared to $1,036.50/percent overBMI for PC-1 and $973.98/percent overBMI for PC-2. Parent cost-effectiveness was $132.97/pound (lb) for FBT and $373.53/lb (PC-1) or $351.00/lb (PC-2). CONCLUSIONS: For families with overweight/obese children and parents, FBT presents a lower cost per unit of weight loss for parents and children than treating the parent and child separately. Given the high rates of pediatric and adult obesity, FBT may provide a unique cost-effective platform for obesity intervention that alters weight in overweight/obese parents and their overweight/obese children.