Cranial computerized tomography (CCT) of 3 children with nephropathic cystinosis and chronic renal failure (CRF) revealed a hydrocephalus internus and externus. In two boys the findings consisted of bilateral dilatation of the ventricular system and of the subarachnoid space; in one boy the alterations were mainly unilateral. The children had repeated convulsions which could not be explained by deterioration of renal function. Their neurological condition was otherwise normal. In six non-cystinotic patients with chronic renal failure, CCT showed normal anatomical structures. The possibility is discussed that the hitherto unknown pathogenetic mechanism of cystinosis leads to diffuse cerebral atrophy, resulting in internal and external hydrocephalus.