Seventy-nine adult patients with primary open-angle glaucoma (POAG) were randomly assigned to treatment by modified trabeculotomy ab externo (TAB) (n = 44) and by trabeculectomy with adjunctive mitomycin C (TMC) (n = 35), and the postoperative outcomes achieved with these two techniques were compared. With TAB, the probability of successful intraocular pressure control 1 year postoperatively was .8644; with TMC, .8432; the difference is not significant (P = .7956). However, postoperative complications such as corneal epithelial damage, bleb leaks, hypotony, flat anterior chamber, and serous choroidal detachment were significantly less frequent in the TAB group. Largely because of this latter consideration, in selected cases, TAB may be a viable alternative to TMC.