The development of an encapsulated filtering bleb (Tenon's cyst) complicated 56 of 409 consecutive filtering operations (13.7%) performed during a 40-month period after January 1983. Fifteen eyes (27.8% of encapsulated blebs) required surgical revision. The recognition of bleb encapsulation occurred 20.4 +/- 12.7 days (mean +/- standard deviation) postoperatively. Prolonged duration of beta-adrenergic antagonist therapy was associated with an increased frequency of bleb encapsulation (180.6 +/- 128.5 weeks without encapsulation, 229.0 +/- 129.3 weeks with encapsulation, P less than 0.009). Bleb encapsulation occurred in 42 of 272 eyes with previous argon laser trabeculoplasty, but in only 4 of 85 eyes without any previous anterior segment laser (P less than 0.01). Encapsulated filtering blebs developed in 4 of 12 (33.3%) eyes with congenital glaucoma and 4 of 9 (44.4%) eyes with juvenile glaucoma (P less than 0.0002). The intraocular pressures (IOPs) in the eyes with encapsulated filtering blebs were significantly elevated at 1, 2, and 3 postoperative weeks, and at final follow-up compared with eyes without bleb encapsulation.