The authors conducted a randomized, controlled clinical trial to quantify the effect of 5-fluorouracil (5-FU) on filtration surgery. Both eyes of 17 patients with either bilateral low-tension glaucoma or advanced chronic open-angle glaucoma that would be expected to progress at intraocular pressures in the normal range were included. One eye received a full-thickness filtering procedure with a modified shell tamponade technique, and the other received a trabeculectomy with postoperative 5-FU. Follow-up was equal for both eyes and averaged 9.2 months. In eyes in the shell group, the average postoperative pressure was 10.94 mmHg, and in the 5-FU group it was 6.94 mmHg, (P = 0.0001). However, the postoperative visual acuity decrease was significantly greater in the 5-FU group (P less than 0.05). Choroidal detachments also were more frequent in the 5-FU group, but this was not statistically significant.