Results of a phase II study evaluating safety, side effects and therapeutic activity of transurethral microwave thermotherapy in patients with symptomatic benign prostatic hyperplasia, are reported. 221 patients with Madsen symptom score >8, peak flow rate <15 ml/s and post void residual urine <200 ml were enrolled between January 1991 and April 1992. Microwave thermotherapy was carried out in a single outpatient session; up to 60 watts power output were delivered and urethral and rectal temperature alarms set at 44.5 and 42.5°C, respectively. Patient compliance to microwave thermotherapy was satisfactory and iv sedation was required in only one patient. Modest haematuria and urethrorrhagia occurred for a few hours in all patients, after treatment; clot retention, requiring bladder irrigation for 48 hours, occurred in 1 patient. A Foley catheter was positioned after treatment in 32 patients for acute urinary retention and withdrawn after 10.5 days, on average. Urinary tract infections occurred in 11 patients and were successfully treated. Retrograde ejaculation or stenosis of the ejaculatory ducts were observed in 2 out of 84 patients. At one year, Madsen symptom score decreased on average, from 11.80 ± 4.91 to 4.20 ± 4.41, peak flow rate increased from 8.89 ± 4.22 to 12.90 ± 3.75 ml/s, post void residual urine decreased from 132 ± 17.8 to 47 ± 41.4 ml/s. Transurethral microwave thermotherapy appeared to be, in our experience, a safe technique with negligible morbidity: therapeutic activity proved to be satisfactory in selected patients