Objective To assess from a societal perspective the incremental cost-effectiveness of a family-based GP-mediated intervention targeting overweight and moderately obese children. The intervention was modelled on the LEAP (live, eat and play) trial, a randomised controlled trial conducted by the Centre for Community Child Health, Melbourne, Australia in 2002-2003. This study was undertaken as part of the assessing cost-effectiveness (ACE) in obesity project which evaluated, using consistent methods, 13 interventions targeting unhealthy weight gain in children and adolescents.Method A logic pathway was used to model the effects of the intervention compared to no intervention on body mass index (BMI) and health outcomes (disability-adjusted life years--DALYs). Disease costs and health benefits were tracked until the cohort of eligible children reached the age of 100 years or death. Simulation-modelling techniques were used to present a 95% uncertainty interval around the cost-effectiveness ratio. The intervention was also assessed against a series of filters ([`]equity', [`]strength of evidence', [`]acceptability', [`]feasibility', sustainability' and [`]side-effects') to incorporate additional factors that impact on resource allocation decisions.Results The intervention, as modelled, reached 9685 children aged 5-9 years with a BMI z-score of >=3.0, and cost $AUD6.3M (or $AUD4.8M excluding time costs). It resulted in an incremental saving of 2300 BMI units which translated to 511 DALYs. The cost-offsets stemming from the intervention totalled $AUD3.6M, resulting in a net cost per DALY saved of $AUD4670 (dominated; $0.1M) (dominated means intervention costs more for less effect).Conclusion Compared to a [`]no intervention' control group, the intervention was cost-effective under current assumptions, although the uncertainty intervals were wide. A key question related to the long-term sustainability of the small incremental weight loss reported, based on the 9-month follow-up results for LEAP.