The fetal overnutrition hypothesis proposes that greater maternal adiposity results in increased obesity throughout life in the offspring. The authors examined the associations between parental prepregnancy body mass index (BMI; weight (kg)/height (m)(2)), based on height and weight reported by the mother at her first antenatal clinic visit, and offspring BMI (height and weight measured at age 14 years) in 3,340 parent-offspring trios from a birth cohort based in Brisbane, Australia (mothers were recruited in 1981-1984). The maternal-offspring BMI association was stronger than the paternal-offspring BMI association. In the fully adjusted model, the increase in standardized offspring BMI at age 14 for a one-standard-deviation (SD) increase in maternal BMI was 0.362 SD (95% confidence interval: 0.323, 0.402), and the corresponding result for a one-SD increase in paternal BMI was 0.239 SD (95% confidence interval: 0.197, 0.282). There was statistical support for a difference in the magnitude of the association between maternal-offspring BMI and paternal-offspring BMI in all confounder-adjusted models tested (all p's < 0.0001). In sensitivity analyses taking account of different plausible levels of nonpaternity (up to 15%), the greater maternal effect remained. These findings provide some support for the fetal overnutrition hypothesis.