We have used transurethral microwave thermotherapy in the treatment of benign prostatic hyperplasia since October 1991. Irreversible cell damage occurs when the microwaves heat the periurethral prostatic tissue. The urethra is simultaneously cooled, and is not destroyed during treatment. The patients are not hospitalized. Transurethral microwave thermotherapy is performed under local anaesthesia, no other form of analgesia has been found necessary. We have included patients with symptomatic prostatic obstruction who would otherwise have received operative or pharmacologic treatment. Patients with high residual urine, large middle lobe, urethral stricture, prostatic cancer, decreased renal function, urinary infection or metal implants were excluded. We observed a marked improvement in the Madsen symptom score after transurethral microwave thermotherapy. The score decreased from 12 before treatment to five after six weeks and 4.6 after six months (p < 0.0001). Flow increased from 8.9 to 9.7 ml/s. There was a significant reduction in the residual urine from 102 ml preoperatively to 69 ml after six months (p < 0.001). The volume of the prostate was only slightly reduced after transurethral microwave thermotherapy. Postoperative edema caused urinary retention in 13% of the patients. Two patients required transurethral resection of the prostata.