Despite the fact that trabeculectomies performed with Mitomycin C (MMC) have a high rate of success as repeatedly reported, the rate of complications encountered draws much of the attention of this substance to those cases that are unsatisfactory. To ascertain a possible relationship between some of these complications and concentrations of MMC used, we performed, in a prospective study, trabeculectomies for complicated cases of glaucoma with either concentrations of 0.5 mg/ml of MMC (group 1) or 0.2 mg/ml of MMC (group 2). The two groups were followed-up for 6 months and consisted of 49 and 40 cases, respectively. Two eyes in the first group and 3 in the second had to be considered as surgical failures that developed increasingly high pressures. Complications included hypotony most frequently (33%, both groups) with severely decreased vision in 3 cases (6%) of the first group and 8 (20%) in the second. Other results including visual acuities, IOP, and the rate of other complications were not significantly different in both groups and indicate that there may be no large difference between the two concentrations of MMC used.