The purpose of this study was to evaluate the performance of bioimpedance analysis (BIA) in the prediction of total body water and fat free mass with the use of standard equations in assessing 20 prepubertal children infected with human immunodeficiency virus (HIV). Total body water was measured by means of deuterium oxide dilution, fat free mass by means of total body dual X-ray absorptiometry, and BIA with a bioelectrical impedance analyzer. The use of standard prediction equations resulted in substantial error. Regression equations using height and BIA resistance for estimating total body water and fat free mass were developed and appear to improve accuracy for prediction. This study suggests that total body water and fat free mass can be estimated in children with HIV by means of BIA equations specifically developed for use with this group of children.