OBJECTIVE: Many patients has suffered from intradialytic hypo- or hypertension. These unsatisfactory control of blood pressure(BP) may be due to inadequate ultrafiltration during HD. In this study, segmental multifrequency BIA was used to estimate dry weight(DW). Then, ultrafiltration adequacy was evaluated according to the estimated DW. METHODS: Total body water resistance(RTBW) and extracellular fluid resistance(RECF) were measured on the right lower extremity in 36 maintenance HD patients(22 men and 14 women) and in 234 control subjects(171 men, 63 women). BP was measured every 30 minutes during HD. Patients with RTBW/ RECF ratio higher and lower than the mean+/-2SD of control subjects were considered to be over- and underhydrated, respectively. RESULTS: 1)RTBW/RECF ratio of control subjects was 0.3402+/-0.0062. 2)RTBW/RECF ratio of HD patients was 0.3650+/-0.0190 before and 0.3445+/-0.0150 after dialysis(P< 0.01). After dialysis, 28 out of 36(77.8%) patients were normovolemic(NV group), 7(19.4%) patients were overhydrated(OV group), and 1 patient(2.8%) was underhydrated. 3)BP changes during HD; In NV group, BP were increased significantly(P<0.05). But there was no change in OV group. In one underhydrated patient, a decrease in BP, albeit not significant, was noticed. 4)Body weight reduction rate(kg/hr) was higher in NV group(0.48+/-0.15) than in OV group(0.25+/-0.14)(P<0.01). CONCLUSION: Segmental multifrequency BIA may be useful as a method for estimating DW and evaluating ultrafiltration adequacy in maintenance HD patients.