Ninety-three consecutive trabeculectomies in which mitomycin C (MMC) was applied intraoperatively, either at a concentration of 0.5 mg/ml for 5 minutes (n = 48) or at a concentration of 0.4 mg/ml for 3 minutes (n=45), were reviewed to determine whether there was a significant difference between the two groups in terms of intraocular pressure (IOP) control and the incidence of complications. Preoperative and postoperative IOPs in the two groups did not differ significantly (P = .817 and .131, respectively). The 5-minute group had a higher incidence of serous choroidal detachments (P =.001) =.001). The incidence of postoperative hypotony and filtration failure with uncontrolled IOP in the two groups did not significantly differ (P =.177 and .582, respectively). Thus, intraoperative application of MMC at a concentration of 0.4 mg/ml for 3 minutes controlled postoperative IOP as effectively as MMC at a concentration of 0.5 mg/ml for 5 minutes, with a significantly lower incidence of choroidal detachments.