This study was designed to compare the clinical results of an ab externo approach to laser sclerostomy using two different laser sources under identical conditions. A pulsed (200 microseconds) erbium-YAG laser (lambda = 2940 nm) and a pulsed (200 microseconds) holmium:YAG laser (lambda = 2120 nm) were used. The energy of each laser was transmitted via a fiber (300 microns in diameter) and applied near the limbus, with the fiber tip remaining in steady contact with the sclera. According to the higher tissue absorption coefficient, a shorter penetration depth and, therefore, fewer complications were expected for the Er:YAG laser. A total of 26 patients with advanced open-angle glaucoma were treated and followed for up to 6 months. In all cases a functioning fistula with a prominent filtering bleb and a marked reduction in the intraocular pressure (from up to 35 mm Hg to < 10 mm Hg) could be achieved primarily. The total energy required was about 4 times lower for erbium:YAG laser was compared with holmium:YAG laser sclerostomies. No complication occurred intraoperatively. Postoperatively, reversible adherence of the iris to the internal ostium was more frequently observed in Ho:YAG laser sclerostomies (60%) and small hyphemas were more often seen in Er:YAG laser sclerostomies (30%). After 6 months of follow-up, about 40% of the fistula remained patent in both groups. In principle, both lasers are suited to serve as energy sources for the described approach. Under the conditions of the present study, different levels of thermal effects did not cause any significant difference in the clinical success rate.(ABSTRACT TRUNCATED AT 250 WORDS)