Left-handedness is most often genetic, but may also follow early, localised damage to the developing brain. This so-called "pathological" left-handedness syndrome is often associated with right hemisphere speech dominance. To find out whether verbal laterality or handedness were affected by congenital, intracranial arachnoid cysts, 51 consecutive patients with temporal or frontal arachnoid cysts were tested for handedness and verbal laterality, as measured with the dichotic listening (DL) technique. Handedness was normal in all subgroups of patients. In the preoperative DL test, only 51 % of the patients showed the normal superiority of the right ear (Right Ear Advantage - REA), significantly different from the REA frequency (74 %) in a normal reference group. Patients with a left temporal, or a frontal cyst had significantly lower preoperative REA frequencies than the reference group, whereas patients with a right temporal cyst did not differ from the reference group. Three to six months after decompressive surgery, the REA frequency (73 %) in the cyst patients was no longer different from that of the reference group. This postoperative normalization was seen both in patients with a left temporal or a frontal cyst. It is concluded that arachnoid cysts may suppress cognitive, cortical functions, and that this suppression can be reversed by surgical decompression of the cyst, even in adults. In our opinion, this cognitive "normalization" may in itself be an indication for decompressive surgery. It is further concluded that, although such cysts are congenital, the pressure from the cyst on the adjacent brain is not strong, nor persistent enough to cause a pathological left-handedness.