This is a prospective pilot investigation of interstitial laser treatment. Twelve patients were treated at 13 sites: seven patients had metastatic or primary liver cancer (with a total of 21 tumour nodules), two had pancreatic carcinoma and four patients had disease at other sites. Treatments were performed with an Nd-YAG laser, using a high power (6 or 10 W), short-time (5 min) technique or a feedback system for temperature regulation at low power (3 W) for 12-16 min. Treatment with high power invariably resulted in rapid carbonization of tissue, which may have contributed to the postoperative death in one patient. The local effect of treatment could be evaluated in 13 hepatic tumours (1.0-10 cm in diameter): 100% necrosis was seen in five and >50% necrosis in the remaining eight. Two tumours were eradicated, five became smaller, and six remained unchanged in size or showed continued growth. Treatment removed or alleviated symptoms in 7/8 symptomatic patients. The feedback system made it possible to avoid carbonization and allowed better control of the tissue temperature. The main problem with either method was to monitor tissue changes in real time, and ultrasonography was found to be of little help in this respect. It is concluded that interstitial laser treatment is a promising method for treatment of tumours. Further development should focus on real-time monitoring and increased volume effect without carbonization.