Between April 1993 and May 1994, 66 patients with benign prostatic hyperplasia (BPH) were treated with transurethral balloon laser thermotherapy. No significant difference was observed in the prostatic volume between the baseline and the volume at one month after treatment. However, at 3 months and at 6 months, there was clearly a significant reduction in prostatic volume compared to the baseline (p<0.0001). The mean reduction rates of the prostatic volume were 12% at 3 months and 16% at 6 months respectively. The degenerative necrosis of the prostatic tissue was clearly observed at 2 weeks after treatment, using light and electron microscopy. The serum prostatic-specific antigen increased to a level 4 times that of the baseline on the 7th day. Noradrenaline-induced peristaltic pressure elevations of the prostatic urethra were significantly inhibited 3 and 6 months after treatment. The prostatic volume reduction and the inhibition in responsiveness to noradrenaline in the prostatic urethra may play a role in a reduction of urethral resistance in BPH.