OBJECTIVE Mitomycin-C is effective for improving success rates of trabeculectomies. The authors sought to determine longterm success rates and complications by studying two widely used concentrations. METHODS Trabeculectomies were performed with mitomycin-C (MMC) in concentrations of 0.5 mg/ml (group 1) and 0.2 mg/ml (group 2) in a prospective, nonrandomized fashion. Patients were observed for at least 3 years, and all follow-up data were analyzed in addition to the preoperative and postoperative conditions and complications. RESULTS A total of 51 and 40 eyes (groups 1 and 2, respectively) were observed for 3 years. The majority of the eyes had repeat trabeculectomy. Complete surgical success was reached in 76% and 63%, respectively. The mean intraocular pressure (IOP) in the two groups decreased significantly, and the decrease was larger in the MMC 0.5 mg/ml group (P < .015). The mean visual acuity did decrease in the two groups, but this was not significant. Complications included short-term and long-term hypotony, conjunctival dehiscence, choroidal detachments, and endophthalmitis. Trabeculectomies failed both within the first 3 months (n = 4; 4%) and later (n = 11; 12%). CONCLUSIONS Three-year results of trabeculectomies performed with MMC are promising. A dose-response effect of the concentration of MMC and IOP levels is present, suggesting that the higher concentration leads to lower IOP values. Despite the antimetabolic effect of MMC, trabeculectomies fail in both the short-term and the long-term.