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.