The purpose of the present study was to evaluate the long-term results of lower-energy transurethral microwave thermotherapy (TUMT) and to determine predictors for a favorable treatment outcome in an international multicenter study. A total of 1092 patients treated between April 1990 and September 1993 in 6 different centers in different countries were evaluated. All patients were treated in a nonblinded, noncontrolled fashion with the Prostatron thermotherapy device using the lower-energy treatment protocol Prostasoft 2.0. Collected data included voiding parameters, Madsen symptom scores, retreatments, types of retreatment, and dates of retreatment. Instrumental retreatment served as the end point for further evaluation. The average age of our patients was 67 years. At baseline the average uroflow rate was 8.7 ml/s. After treatment the improvement in uroflow was 2-3 ml/s. This was maintained for up to 5 years after treatment for the patients remaining in follow-up. The overall improvement in the Madsen symptom score was 5-6 points for these patients. There was no significant difference between the different centers. During follow-up, however, the number of patients remaining in follow-up decreased rapidly. The absolute instrumental retreatment rate appeared to be 26%; however, when patients no longer in follow-up were taken into account, the calculated retreatment rate was 39.6% (Kaplan-Meier survival analysis). Patients undergoing retreatment were younger at baseline and had a higher Madsen score, a bigger prostate, and a greater postvoid residual. No major complication was seen. Lower-energy TUMT gives a sustained objective and subjective improvement in patients with moderate symptoms and a low-grade bladder outflow obstruction. Patients with bigger prostates, severe symptoms, low rates of maximal uroflow, and large residuals are prone to have a higher degree of prostatic obstruction and are not the ideal candidates for this treatment. The absolute instrumental retreatment rate after 5 years was 26%. Moreover, no significant international difference in treatment outcome was found.