Cysts of Tenon's capsule (encapsulated blebs) developed postoperatively in 77 (13%) of 607 eyes that underwent filtration surgery between 1980 and mid-1985. The annual incidence was not uniform, increasing markedly throughout the period. Seventy-four eyes were treated with medical therapy only. At a mean follow-up of 19 months, the success rate for these eyes, defined as an intraocular pressure (IOP) of 21 mm Hg or less, was 92%. The three eyes that underwent surgical revision subsequently failed clinically by the study criteria. Those patients in whom cysts of Tenon's capsule developed were compared with an age-matched control group to assess for possible differences in long-term outcome. There was a significantly higher IOP in the Tenon's cyst group at one and three months after surgery, and an increased proportion of IOPs above 30 and 40 mm Hg. At a six-month and later follow-up, there was no significant difference in the mean IOP or in progression of visual field loss. Causative factors were sought for the development of these cysts of Tenon's capsule. Prior conjunctival surgery, or previous cyst formation in the other eye, were the significant risk factors noted.