The purpose of this study was to determine the accuracy of standard methods for estimating body composition in individuals who have undergone substantial and rapid weight loss. Subjects included 14 participants in a rapid weight reduction program using a very low-calorie diet, and 14 individuals of normal body composition matched with the obese group on the basis of gender, age, and height. Bioelectrical impedance (BI) and skinfold-derived estimates of relative body fat were compared with hydrostatic weighing in both groups. On the basis of mean values, standard errors of estimate (SEE) and simple correlations, none of the skinfold equations or the BI procedure were very accurate in the prediction of body fat in the group of weight-reduced obese patients. The Durnin-Rahaman (D-R) and Durnin-Womersley (D-W) equations were not significantly different from the hydrostatically determined values; however, correlations were low (r = 0.69 and 0.60). The Jackson-Pollock (J-P) equations and BI technique had high correlations (r = 0.78 and 0.89), but were significantly different from hydrostatic weighing. SEE for all equations and BI were high (greater than 4.5%). The control group was best predicted by the J-P equation (r = 0.88, SEE 3.5%, mean value of 21.2 +/- 4.3 vs 22.4 +/- 6.2%). BI and the D-R equation were also reasonably accurate. We conclude that additional research will be necessary to identify a predictive equation using skinfolds or BI that will provide more accurate estimates of relative body fat in individuals who have undergone substantial weight reduction in a relatively short period of time.