To determine whether the effectiveness of oxytocin treatment of arrest of dilatation differed in obese compared to lean women, we did a retrospective analysis of 118 subjects in spontaneous labor whose arrest of dilatation was treated with oxytocin. Cases were stratified by maternal body mass index (BMI): Group A, <25 kg/m2; Group B, 25.0–29.9 kg/m2; Group C, 30.0–34.9 kg/m2; and Group D, ≥35 kg/m2. Groups were comparable in maternal age, parity, gestational age, birth weight, and the frequency of infection. Full dilatation was reached in about 90% of Group A and B, 72% of Group C, and 39% of Group D, the most obese women (P<0.001). The cesarean rate was directly related to maternal BMI, 11.4%, 22.9%, 34.3%, and 69.6% in Groups A through D, respectively (P<0.001). Significantly more oxytocin was used in group D than in the other groups during the first 3h of infusion in attempting to overcome the arrest (P=0.003). We conclude that oxytocin treatment of arrest of dilatation was less effective in obese than in lean women. This effect was most prominent in women with a BMI >35 kg/m2, who were, despite having received more oxytocin than those in the leaner groups, less than half as likely to attain full dilatation and more than twice as likely to deliver by cesarean.