Body Mass Index (BMI) is used to evaluate overweight and obese status. Correlations between BMI and percent body fat (%BF) are high (r=.6–.8), but BMI may fail to identify certain at-risk individuals. PURPOSE The purpose of this study was to evaluate the use of a unigender BMI to define obesity. METHODS Subject data were obtained from a cardiovascular risk assessment database including active adults without a history of cardiovascular disease. Height, weight and %BF by hydrodensitometry were obtained on all participants. Data collection occurred between 1983 and 1998. Obesity is defined, by hydrodensitometry, as > 25% body fat for men and > 30% body fat for women. RESULTS %BF and BMI were measured in 2591 white men and 634 white women aged 18–65. Mean BMI for non-obese men (25.3 kg/m2) and women (21.7 kg/m2) were different (t=24.03, p < .001), as were the mean BMI for obese men (29.8 kg/m2) and obese women (26.2 kg/m2) (t=12.35, p < .001). BMI and %BF were highly correlated (r=.70 for males, r=.71 for females). Using a BMI of < 25 kg/m2 as an indicator of normal weight status, 12% of men and 27% of women with a normal BMI were obese by percent body fat. Increasing age accounted for a significantly larger proportion of those misclassified. CONCLUSION Body mass index is influenced by gender and age in Caucasians and argues against the use of a uniform BMI for determination of overweight and obesity. Furthermore, despite the correlation between BMI and percent body fat, approximately 1 in 10 men, and 1 in 4 women with a normal weight status by BMI are obese and may not receive appropriate advice regarding weight loss.