Purpose To evaluate intraocular penetration of topical fluorescein in eyes with avascular blebs after trabeculectomy. Design Case control study. Methods The study included 11 eyes with open-angle glaucoma and functioning avascular blebs, six of which were treated with topical antiglaucomatous medications and had no history of surgery, 15 with open-angle glaucoma, and untreated eyes suspected of having open-angle glaucoma. The fluorescein concentration in the superior peripheral and central corneal stroma and anterior chamber was determined 30 and 60 minutes after fluorescein instillation. Results The fluorescein concentration in the superior cornea was significantly higher in eyes with blebs or those topically treated than in untreated eyes ( P <.01); there was no significant difference in the central cornea. The fluorescein concentration in the anterior chamber was much higher in eyes with blebs than in those that were untreated or topically treated ( P <.001). Conclusion The presence of avascular filtering blebs greatly enhances intraocular penetration of topically instilled fluorescein.