Ab externo thermal sclerostomy was performed with the holmium YAG (thulium, holmium, chromium-doped YAG crystal) laser in 21 eyes of 20 patients with refractory glaucoma. We used either the 5-fluorouracil (5-FU) or mitomycin C (MMC) antimetabolites in all cases. The rate of intraocular pressure (IOP) control defined as IOP < 21 mmHg regardless of whether antiglaucoma medication was applied postoperatively) was estimated by the life-table methods of Kaplan-Meier. The postoperative IOP control rate was 47.1% in the MMC-treated group at 57 months, and 14.3% in the 5-FU-treated group at 52 months. There were statistically significant differences in success rates between the MMC- and 5-FU-treated groups. We observed no clinically significant complications except excess filtration associated with a shallow anterior chamber in one case. This procedure is thought to have several advantages over more conventional filtration surgery; the operation time is shorter, and there is no need for intraocular manipulation, which means conjunctival trauma is minimal. However, the IOP control rate was substantially lower than that achieved via conventional trabeculectomy. Our results suggest that the selection of patients and the use of MMC is an important factor in maintaining successful filtration.