Since the urea kinetic model is used as an objective method for monitoring dialysis, it has been possible to shorten reasonably its duration. From the usual practice, we have observed that it is easier to reduce the dialysis time in women as compared to men. The purpose of this study is to find out the reason for such observation and to corroborate it. Fourty-two patients, 25 males and 17 females on 3-weekly dialysis (3 h/session) were studied. All patients were dialyzed under the same dialysis characteristics: 3-hour sessions, blood flow 350 ml/min, ultrafiltration 1.5 liters/h, 1.5-m2 cuprophane membrane and bicarbonate buffer. The dialysate was collected in a graduated tank. Urea concentration in plasma and in the dialysate was measured. Then, the urea distribution volume, dialyzer clearance, the KT/V index and protein catabolic rate were calculated. The KT/V value was higher in women with respect to men, 1,017 ± 0.10 versus 0.82 ± 0.14 (p < 0.001). The urea distribution volume value was significantly higher in men as compared to women, 60.04 ± 6.6 versus 51.48 ± 5.88% (p < 0.001). There were no significant differences in dialyzer clearance, protein catabolic rate or body weight. In conclusion, under identical dialysis conditions, it is easier to dialyze women than men, because women’s urea distribution volume is lower than