Malnutrition is common in patients with acquired immunodeficiency syndrome (AIDS), which distorts the chemical contents in the fat-free mass (FFM) and alters the assumptions underlying the traditional methods for calculating body-fat content so that such measurements may not be accurate. In vivo neutron-activation analysis (IVNA) measures FFM independently of the traditional assumptions, thereby providing more accurate measurements of body fat. We compared seven methods for measuring body fat in 18 male patients with AIDS: IVNA, total body water (TBW by 3H2O dilution), total body potassium (TBK by 40K counting), dual-photon absorptiometry (DPA), bioelectrical impedance analysis (BIA), and two well-calibrated anthropometric methods. FatTBW and fatDPA were not significantly different from fatIVNA. FatTBW gave the highest correlation with fatIVNA and the smallest SEE of +/- 1.8% (1.1 kg). The traditional and widely available TBW and the newer DPA method provide reliable estimates of fatIVNA in patients with AIDS.