In the present study we report the results of Nd:YAG laser treatment in 36 patients with rectal carcinoma in whom negative biopsies were obtained at the end of the treatment. The laser (100-W maximal power output) was applied through a flexible endoscope during 20- to 40-min sessions repeated every three days until the lesion was destroyed completely. Follow-up examinations, including endoscopy with biopsies, liver and endorectal ultrasonography and chest x-ray were performed every three months during the first year and thereafter once a year. Between 1980 and 1991, 272 patients were treated. All were unfit for surgery because of metastasis (78), recurrence after an other procedure (54), associated conditions, or old age (140). No circumferential tumors of any size were obliterated, but among the 139 patients presenting with a noncircumferential lesion less than 7 cm in diameter, negative biopsies were obtained after laser treatment in 36 patients (26%). Of these 36 patients, eight had been treated previously by surgery (5) or radiotherapy (3). Mean follow-up is 37 months (range 12-71). Recurrences were observed in four cases. Seven patients died during the study but only one death was related to the cancer (pelvic extension 19 months after treatment). Endorectal ultrasonography was performed prior to treatment in 15 patients and showed no invasion of the rectal wall deeper than the submucosa. After treatment, endorectal ultrasonography in 22 patients showed significant changes corresponding to cicatricial pattern in 60% of controlled patients.(ABSTRACT TRUNCATED AT 250 WORDS)