We examined body mass index (BMI) as a proxy for percent body fat among 26 men and women successfully undergoing orthotopic heart transplantation. Percent body fat was determined by use of bioelectrical impedance techniques. We found that, although BMI was well correlated with percent body fat (r = 0.58, p < 0.01), use of a BMI of greater than 27 kg/m2 to define obesity potentially misclassified patients when compared with defining obesity as a percentage of body fat as both greater than 30% (BMI = 9 of 26 patients vs percent body fat = 6 of 26 patients) and greater than 40% (BMI = 9 of 26 versus percent body fat = 1 of 9). We conclude that percent body fat measurements are more methodologically appropriate means for J Heart Lung Transplant 1997;16:563-5.