It has recently been suggested that there are substantial differences in mother-daughter and father-son associations of body mass index and obesity among contemporary UK children, but much larger studies of older cohorts have failed to find evidence of substantial sex-specific effects. We have tested this hypothesis using the Avon Longitudinal Study of Parents and Children, a large contemporary cohort. Our analyses are based on 4654 complete parent-offspring trios (2323 with male offspring and 2331 with female offspring, all aged approximately 7.5 years). We found maternal body mass index to be a little more strongly associated with female than with male offspring body mass index (beta=0.18 (95% confidence interval 0.16-0.20) for females vs 0.13 (0.12, 0.15) for males). However, associations between paternal body mass index and male compared with female offspring were very similar (beta=0.16 (0.14, 0.19) for females vs 0.15 (0.12, 0.17) for males). Hence, our study suggests that there is no compelling reason to integrate the belief that there are large differences in parent-offspring body mass index associations with obesity prevention strategies.