There are relatively few reports that evaluate the cognitive functions of patients with arachnoid cysts. Presumably, these 'silent cysts' are regarded as incidental findings with no functional significance. Although postoperative clinical improvement is well documented in patients with significant reduction in cystic volume, the current report describes a patient who underwent cystoperitoneal shunting due to mass effect, with minimal postoperative decompression. Neuropsychological testing indicated significant cognitive improvement in verbal learning, memory, visual-perceptual abilities, constructional skills, conceptual shifting, and psychomotor speed after shunt placement, despite marginal evidence of decompression. These findings suggest that (1) significant cognitive changes can occur in these patients, despite minimal postoperative regression of the lesion, (2) cognitive measures may provide an alternative, functional index of outcome efficacy, and (3) reliance on traditional outcome measures (i.e. anatomical decompression or resolution of clinical symptoms) may underestimate the efficacy of surgical intervention for these patients.