We report the preliminary results of the first 20 consecutive cases in which the antimetabolite 5-fluorouracil (5-FU) was applied directly beneath the conjunctival flap during trabeculectomy in high-risk patients. Seventeen were considered early successes, with 3-month postoperative intraocular pressures less than 21 mmHg, representing at least a 20% decrease from preoperative values. Successful blebs were pale, with conjunctival microcysts and without significant vascularization over the trabeculectomy site, similar to the appearance of blebs in eyes administered postoperative subconjunctival injections of 5-FU. There were no cases of corneal epithelial defects, and no eyes had lost more than one line of preoperative vision at the time of last follow up. Although the longer term efficacy of this method is unknown, these results suggest that it may represent a safe and effective alternative for administering 5-FU.