Purpose: To determine the effect on the corneal endothelium Of mitomycin C (MMC)-augmented trabeculectomy 3 and 12 months after surgery. Patients and Methods: In a prospective, observational clinical study, 14 eyes of 14 patients with well-established open angle glaucoma were followed for 12 months after MMC-augmented trabeculectomy. We Measured the endothelial cell density, the variation in mean cell size, the percentage of hexagonal cells, and the central corneal thickness at baseline and 3 and 12 months postoperatively. Results: The mean endothelial cell density was 2525 [standard deviation (SD) 457] cells/mm(2) before Surgery and 2332 (SD 643) cells/mm(2) 2 and 2283 (SD 597) cells/mm2 at 3 and 12 months after surgery, respectively, representing a cell loss of 9.5% (P = 0.052) and 10.0% (P = 0.027). The cell loss from 3 to 12 months was nonsignificant (P = 0.40). After 12 months of follow-up, the variation in mean cell size and the percentage of hexagonal cells had not yet stabilized around the preoperative values. Conclusions: Our results show that significant cell loss occurs during or immediately after MMC-augmented trabeculectomy However, no progressive cell loss was observed from 3 to 12 months, demonstrating that MMC has no prolonged toxic effect on the corneal endothelium. From 3 to 12 months after MMC-augmented trabeculectomy, active endothelial adaptations were taking place.