OBJECTIVE: To investigate the genetic and environmental influences on waist-to-hip ratio (WHR) and waist circumference (WC) measurements in males and females. DESIGN: Measurements taken from 1989-1991 as part of The Swedish Adoption/Twin Study of Aging (SATSA) were used for analysis. The SATSA sample contains both twins reared together as well as twins reared apart. SUBJECTS: 322 pairs of twins (50 identical, 82 fraternal male pairs and 67 identical, 123 fraternal female pairs); age range: 45-85y (average age, 65y). MEASUREMENTS: Waist-to-hip ratio (WHR), waist circumference (WC) and body mass index (BMI). RESULTS: In males, additive genetic effects were found to account for 28% of the variance in WHR and 46% of the variance in WC. In females, additive genetic effects were found to account for 48% of the variance in WHR and 66% of the variance in WC. The remaining variance in males was attributed to unique environmental effects (WHR, 72%; WC, 54%) and in females the remaining variance was attributed to unique environmental effects (WHR, 46%; WC, 34%) and age (WHR, 6%). When BMI was added into these models it accounted for a portion of the genetic and environmental variance in WHR, and over half of the genetic and environmental variance in WC. CONCLUSION: There are both genetic and environmental influences on WHR and WC, independent of BMI in both males and females, and the differences between the sexes are significantly different.