A THC:YAG laser (thulium, holmium, chromium-doped YAG crystal) was used to create 93 thermal sclerostomies ab externo in 81 glaucomatous eyes of 76 patients. Pulse energies of 80 to 120 mJ were used, with a repetition rate of 5 pulses per second. Total energy levels to produce full-thickness sclerostomies ranged from 1.4 to 7.2 J. Estimated probability of success was 0.66 at 12 months and 0.57 up to 30 months. The mean intraocular pressure in the successful cases was 12.8 +/- 4.0 mm Hg.