We report our early experience in the clinical application of interstitial laser coagulation of the prostate (ILCP) in the treatment of benign prostatic hyperplasia (BPH). Neodymium: YAG laser energy is transmitted via a specially designed interstitial thermotherapy light-guide. The light guides were inserted transurethrally into each lobe of the prostate by direct puncture, under direct visualization. The prostatic urethra is preserved during the procedure. From December 1993 to March 1994, 37 patients with symptomatic BPH were treated with ILCP. Treatment outcome was evaluated by the International Prostatic Symptom Score (I-PSS), flow rate, postvoided residual urine volume and quality of life score. Significant improvement in I-PSS and peak flow rate was observed at 3 months: I-PSS decreased from a mean of 19.3 to 10.4, and the peak flow rate increased from a mean of 7.2 to 9.4 ml/sec (p < 0.0001). The mean postvoided residual urine volume significantly decreased from 91 to 47 ml (p < 0.01). Two days after ILCP the serum prostate-specific antigen had increased by 860%, evidence of the significant tissue damage produced by laser irradiation. The quality of life score significantly decreased from a mean of 4.8 to 2.1 (p < 0.0001). No serious side-effects were observed. The early clinical results suggest that ILCP is safe and effective as a treatment of BPH and is less invasive than some other methods.