Bioelectrical impedance analysis (BIA) is composed of resistance (R) and reactance (Xc). The aim of this study was to investigate whether BIA may be influenced by the duration of hemodialysis (HD) sessions. Eleven uremic patients underwent one 4-hour and one 8-hour bicarbonate HD session. Volume of blood and dialysate processed, volume of ultrafiltration (V(UF)), and dialysate electrolyte concentrations were prescribed to be the same. R and Xc were determined at the start and the end of each session, injecting 800 μA at 50 kHz alternating sinusoidal current (BIA 101; Akern, Italy). Mean pre- and postdialysis body weights and V(UF) were not significantly different in the 4-hour and 8-hour treatments. Postdialysis R, ΔR (the difference between post- and predialysis R values), and percent increase of R values were significantly higher in the 8-hour sessions, when compared with the corresponding values of the 4-hour sessions (p < 0.0001, 0.02, and 0.02, respectively). In conclusion, this study shows that 8-hour HD sessions were associated with postdialysis R, ΔR, and percent increase of R values significantly higher than the corresponding ones of 4-hour sessions. If higher R values may represent a proxy of a correct dry body weight, it remains a matter of future research.