The effect of 5-fluorouracil (5-FU) subconjunctival injection on the bleb formation and intraocular pressure (IOP) following trabeculectomy was studied in 27 primary open-angle glaucoma patients (33 eyes) who had never been operated upon (25 eyes) or had undergone single trabeculectomy that failed to reduce IOP (8 eyes). The results were analyzed by means of life table analysis and compared with those of 65 primary open-angle glaucoma eyes that had undergone trabeculectomy without postoperative administration of 5-FU as the first or the second filtering surgery. The surgical techniques and postoperative care were virtually identical between the eyes treated with 5-FU and the eyes that had undergone trabeculectomy without 5-FU. At the end of 28-month follow-up, the success probability was 74.8% without any postoperative antiglaucoma medication, and 100% with antiglaucoma medication. Whereas, the success probability with postoperative antiglaucoma medication was as low as 60.8% for the first, and 51.5% for the second trabeculectomy without 5-FU at the 24-month follow-up. Postoperative, subconjunctival injection of 5-FU appears to improve the prognosis following trabeculectomy in primary open-angle glaucoma patients.