OBJECTIVE To determine the prevalence of late filtering bleb leakage after trabeculectomy performed with intraoperative adjunctive use of 5-fluorouracil (5-FU) or mitomycin C. METHODS Case series. METHODS Private clinic in São Paulo. METHODS Forty-seven consecutive patients (47 eyes) who had previously undergone trabeculectomy with intraoperative application of either 5-FU (25 mg/mL) or mitomycin C (0.2 mg/mL) and who had functioning filtering blebs. The patients who received 5-FU had advanced primary open-angle glaucoma without previous surgery; those who received mitomycin C had either previous failed filtering surgery or refractory glaucoma. All patients had been followed for at least 6 months. METHODS Slit-lamp appearance of bleb, bleb leakage, as determined with the Seidel test. RESULTS None of the eyes had spontaneous bleb leakage. Ten (32.2%) of the 31 eyes that received 5-FU and 5 (31.2%) of the 16 eyes that received mitomycin C had induced bleb leakage. Thirty filtering blebs were classified as transparent, 10 as vascularized and 7 as ischemic. The rates of bleb leakage for the three groups were 46.7%, 0% and 14.3% respectively (p = 0.012). CONCLUSIONS The importance of late bleb leakage in patients who receive intraoperative 5-FU or mitomycin C in predisposing these eyes to late complications, such as endophthalmitis and hypotony, needs to be evaluated.