BACKGROUND: Bioelectrical impedance analysis (BIA) can be valuable in evaluating the fat-free (FFM) and fat masses (FM) in patients, provided that the BIA equation is valid in the subjects studied. The purpose of the clinical evaluation was to evaluate the applicability of a single BIA equation to predict FFM in pre- and posttransplant patients and to compare FFM and FM in transplant patients with healthy controls. METHODS: Pre- and posttransplant liver, lung, and heart patients (159 men, 86 women) were measured by two methods-50-kHz BIA-derived FFM (FFM(BIA)) by Xitron instrument and DXA-derived FFM (FFM(DXA)) by Hologic QDR-4500 instrument-and compared with healthy controls (196 men, 129 women), aged 20 to 79 years. RESULTS: The high correlation coefficient (r = .974), small bias (0.3 +/- 2.3 kg), and small SEE (2.3 kg) suggest that BIA using the GENEVA equation is able to predict FFM in pre- and posttransplant patients. The study shows that the lower weight seen in transplant men and women than in controls was due to lower FFM, which was partially offset by higher FM in men but not in women. Furthermore, the higher weights in posttransplant than in pretransplant patients were due to higher FM and % FM that was confirmed by lower FFM/FM ratio in posttransplant patients. CONCLUSIONS: Single 50-kHz frequency BIA permits measurement of FFM in pre- and posttransplant patients.