To evaluate the safety and efficacy of thermal balloon therapy for menorrhagia. Prospective, observational study. Fifteen centres in Canada and Europe. Two hundred and ninety-six eligible women for whom follow up data were available for three months or more. Eligible women included those for whom further fertility was not a concern, were not postmenopausal, suffered from intractable menorrhagia, had a normal uterine cavity, and who were fully informed regarding the investigational nature of uterine thermal balloon therapy. Three hundred and twenty-one procedures of balloon endometrial ablation were performed using the same protocol between June 1994 and August 1996. Exclusion criteria included structural uterine abnormality or (pre) malignant lesions. Treatment entailed controlled heating of fluid in an intrauterine balloon. General anaesthesia was employed in the 61% of procedures while local anaesthesia with or without sedation was used in 39% of cases. Follow up data at 3 and/or 6, and/or 12 months were required for inclusion in the analysis. A paired t test, Wilcoxon signed-ranks test, and multiple and logistic regression analyses were used to evaluate the changes in bleeding and dysmenorrhoea patterns, and possible confounding variables, respectively. Success was defined as the subjective reduction of menses to eumenorrhoea or less. No intra-operative complications occurred, and post-operative morbidity was minimal. Success of the procedure was constant over the year (range 88%-91%). Treatment led to a significant decrease in the duration of menstrual flow and severity of pain (P < 0.0001). Increasing age, higher balloon pressure, smaller uterine cavity, and a lesser degree of pre-procedure menorrhagia were associated with significantly improved results. Pre-treatment with gonadotrophin releasing hormone agonists increased amenorrhoea and spotting rates (P = 0.03), but was only used in 5% of cases. Thermal balloon endometrial ablation appears to be safe, as well as effective in properly selected women with menorrhagia and is potentially an outpatient procedure.