Physical activity levels in children are low and factors in the neighbourhood are believed to be influential. However, uncertainty remains about how best to define the neighbourhood. We therefore sought to study the role of area definition on neighbourhood variations in child physical activity using data collected at age 11 from the Avon Longitudinal Study of Parents and Children, UK. We found the effect of neighbourhood of residence on variations in PA was small, explaining under 3% of variance at best, and was not strongly dependent on the manner by which the neighbourhood was defined. Our results suggest that while characteristics of local environments may be important determinants of activity, the delineation of neighbourhoods based on shared social or physical characteristics may not best capture the local influences.