Retrocerebellar arachnoid cysts in siblings with mental retardation and undescended testis.

Arachnoid cysts comprise approximately 1% of all intracranial space-occupying lesions and usually occur sporadically. We report retrocerebellar arachnoid cysts in two male siblings with mental retardation and undescended testis, suggesting the possibility of a genetic basis for at least some cases of retrocerebellar arachnoid cysts.