Dose-inhibition studies of proliferating fibroblasts grown from Tenon's capsule biopsy specimens obtained during trabeculectomy from 17 glaucoma patients were performed using varying concentrations of mitomycin C (MMC), with exposure of cells to the drug for either 5 days or 5 minutes. Surgical outcomes were determined 3 months postoperatively. Based on in vitro results, fibroblasts were classified as either MMC-"resistant" or MMC-"sensitive." Thirteen surgically successful cases were associated with 10 "sensitive" and three "resistant" fibroblasts; four surgical failures with one "sensitive" and three "resistant" fibroblasts. The in vitro studies were positively correlated with the clinical outcomes, the former correctly predicting the latter in 76.5% of the surgeries.