* BACKGROUND AND OBJECTIVE: This study was undertaken to compare the efficacy and safety of low-dose intraoperative application of mitomycin-C (MMC) with that of 5-fluorouracil (5-FU) in primary trabeculectomy. * PATIENTS AND METHODS: A non-randomized prospective study was performed between August 1994 and November 1995. Thirty-two eyes of 16 consecutive patients who underwent trabeculectomy for uncontrolled glaucoma of various causes form the study group. The mean age was 46.8 ± 9.9 years. The first eye received MMC (0.2, 0.4 mg/ml), fellow eye received 5-FU (50 mg/ml), for 1 minute intraoperatively. Bleb characteristics and intraocular pressure (IOP) control were analyzed. Success of surgery based on IOP control was measured by 3 different criteria: IOP less than 21 mm Hg; IOP less than 21 mm Hg with more than 30% reduction; and IOP less than 16 mm Hg with more than 30% reduction. * RESULTS: Mean preoperative IOP was 31.4 ± 12.7 mm Hg in MMC group and 27.8±8.8 mm Hg in 5-FU group. Mean follow-up in MMC group was 16.12 ± 8.17 months; in 5-FU group 13.37 ± 8.19-months. At last follow-up all 5-FU blebs were nonischemic, while 4 eyes in the MMC group showed non-ischemic blebs, and 12 eyes had ischemic blebs. There was no statistically significant difference between MMC group and 5-FU group success rates with all 3 criteria. Success rates were: IOP less than 21 mm Hg; 100% in both groups; IOP less than 21 mm Hg with more than 30% reduction; MMC group 93.8%, 5-FU group 75%; less than 16 mm Hg with more than 30% reduction; MMC group 87.5%, 5-FU group 68.8%. CONCLUSIONS: Low-dose intraoperative MMC and 5-FU can provide control of IOP in primary trabeculectomy, 5-FU group showed more non-ischemic blebs. [Ophthalmic Surg Lasers 2000;31:24-30.]