We used delayed 5-mg subconjunctival injections fo 5-fluorouracil as an adjunct to filtering surgery in ten eyes with refractory glaucoma. The first 5-fluorouracil injection was made three to 15 days postoperatively and was used only when clinical signs suggested an impending bleb failure. These signs included flattening and localization of bleb, increased vascularity, early subconjunctival scarring, loss of microcystic conjunctival changes, and increased intraocular pressure. After a follow-up of ten to 17 months (mean, 13.7 months), and intraocular pressure of less than 21 mm Hg was attained in five eyes without medication and in four eyes with medication. The treatment failed in one eye. The average dose of 5-fluorouracil was 28 ± 12 mg per eye. There were no conjunctival wound leaks. Six eyes developed transient corneal epithelial defects. The modified administration of 5-fluorouracil can result in fewer conjunctival wound leaks. Its use can be limited to those patients who show evidence of impending bleb failure.