The observation in 14 dialysis patients of an encephalopathy associating myoclonia, dysarthria, generalised seizures in some cases, worsening over a few months, led to an aetiological inquiry based upon comparative study of patients with or without encephalopathy treated in the same centre or at home, and controls. Higher levels of aluminium were found in the frontal cortex grey matter of encephalopathy patients as compared to the control group. The same applies to manganese in the white matter. Copper, zinc and iron contents were not different. Aluminium levels in blood, dialysis bath and tap water supply were higher in center dialysis than in home dialysis. Blood aluminium levels at the end of hemodialysis were correlated with bath aluminium levels. The ingestion of alumine gels was not greater in the encephalopathy patients than in other hemodialysis patients; its estimation, in each case, was not related to the blood aluminium levels at the begining of hemodialysis. These finding indicate the need of a routine measure of metal content - mainly aluminium and manganese - in tap water used for dialysis, in order to treat this water if necessary.