We compared the thermocoagulation effects of low power, slow heating (15 W × 180 seconds) versus high power, rapid heating (50 W × 60 seconds) laser regimens in the canine prostate. The study was performed in 20 mongrel canines. On each prostate, the low power regimen was delivered at the 2 and 4 o'clock positions, and the high power at the 8 and 10 o'clock positions. The laser power was measured at the fiber tip. Seven dogs (acute group) were sacrificed 1 hour after the procedure. The other 13 dogs (chronic group) were sacrificed at different time intervals from 3 days to 9 weeks after the procedure. The average depth of coagulation was significantly greater in lesions treated at low power (acute: 10.7 mm., chronic: 13.3 mm.) than in those treated at high power (acute: 8.5 mm., chronic: 11.6 mm.). Another potential benefit of the low power regimen observed in the study was preservation of the integrity and efficiency of the laser probe. © 1995 American Urological Association, Inc.