PURPOSE: To describe the incidence of, and risk factors for, surgical complications reported during and within the first post-operative month after trabeculectomy in the Collaborative Initial Glaucoma Treatment Study (CIGTS). DESIGN: Review of prospectively collected data from a multicenter, randomized clinical trial. METHODS: Complications were tabulated for the 300 CIGTS patients randomized to surgery. Logistic regression analyses were used to identify risk factors for complications. RESULTS: Among the 300 patients randomized to initial surgery, 465 trabeculectomies were performed. Intraoperative complications were reported in 55 eyes (12%). The most frequent reported complications were anterior chamber bleeding during surgery (37 eyes, 8%) and conjunctival buttonhole (five eyes, 1%). Early post-operative complications were reported in 232 eyes (50%). Complications with a frequency over 10% included shallow or flat anterior chamber (62 eyes, 13%), encapsulated bleb (56 eyes, 12%), ptosis (55 eyes, 12%), serous choroidal detachment (52 eyes, 11%), and anterior chamber bleeding or hyphema (48 eyes, 10%). There were three localized suprachoroidal hemorrhages (0.7%) and no cases of endophthalmitis. Older patients were more likely to experience serous choroidal detachment, new anterior or posterior synechiae, and wound leak. Blacks were less likely to experience anterior chamber bleeding, but more likely to experience post-operative ptosis. The number of subjects experiencing bilateral complications was higher than that which would have been predicted by chance alone. CONCLUSIONS: The incidence of transient and self-limiting complications was high in the perioperative period, but we observed few complications with the potential to cause severe sustained vision loss in this group of previously untreated eyes.