The purpose of this study was to compare the effectiveness of 8 versus 16 min of heating in the treatment of menorrhagia with hot fluid balloon ablation. Between 1995 and 1998, women with menorrhagia consecutively scheduled for endometrial ablation with hot fluid balloon were included in a prospective study. The effectiveness of 8 versus 16 min heating time on surgical reintervention and patients' satisfaction was evaluated. Of the 130 women in the study, 63 had endometrial heating for 8 min, whereas 67 patients had endometrial heating for 16 min. Hysteroscopy performed immediately after the procedure showed an evident thermic effect on both the anterior uterine wall and the fundal region in the two groups. The posterior wall was still vital in 25% of the patients after treatment for 8 min, whereas this percentage was only 8% in the 16-min group. This difference was not statistically significant. Failure rates of the hot fluid balloon therapy, defined as either hysterectomy or dissatisfaction with the treatment result, were not significantly different (hazard rate ratio 1.0; 95% confidence interval 0.93-1.2). This-study shows that there is no advantage in doubling the heating period of the balloon ablation therapy from 8 to 16 min.