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Thirteen adult patients were operated on for symptomatic arachnoid cysts in the left temporal fossa; seven with an internal shunt procedure during local anaesthesia, and five with a craniotomy with fenestration of the cyst to the basal cisterns. In one patient, an initial internal shunt was transformed to a cystoperitoneal shunt. After surgery, all patients experienced relief of symptoms. Reduction of cyst volume occurred in 11 patients. The patients were tested for brain asymmetries related to language and verbal memory before and after operation, with a dichotic listening technique with simultaneous presentation of different auditory stimuli to the two ears. In the preoperative memory test, the patients showed impaired total recall compared with healthy control subjects, and recall from the right ear was significantly impaired. The patients also performed poorly in a forced attention task consisting of dichotic presentations of consonant-vowel syllables. In addition to clinical improvement, the surgical procedures led to improvements in both dichotic perception and memory. Overall memory performance was enhanced, mainly because of improved recall from the right ear. This normalisation of memory function was found as early as four hours after the operation. The results indicate that arachnoid cysts in the left temporal fossa may impair cognitive function, that neuropsychological tests are necessary to disclose these impairments, and that cognitive improvement occurs after surgery.