The objective of the study was to determine the efficacy of optic nerve sheath fenestration (ONSF) in eyes with progressive visual or field loss in pseudotumor cerebri in spite of medical therapy with oral Diamox. Visual data on 29 eyes of patients with pseudotumor cerebri who underwent ONSF at Bascom Palmer Eye Institute from 1987 to 1995 were studied retrospectively. These patients had progressive visual loss despite medical therapy. Visual acuity and fields were compared before and after surgery (within 1 and 6 months). During a mean follow-up of 15.7 months (range, 1-50 months), visual acuity improved in four eyes (14%), was unchanged in 22 (76%) eyes, and worsened in three (10%) eyes. Visual fields improved in 10 (48%) eyes, remained unchanged in eight (38%) eyes, and worsened in three (14%) eyes (six were lost in long-term follow-up). There were four repeat surgeries in which vision was lost in one eye. Data from these patients indicate that ONSF improves or protects visual function in patients with pseudotumor cerebri who experience deteriorating visual acuity and fields in spite of medical therapy.