The serum aluminium, zinc, copper, iron and cadmium levels were studied in patients on haemodialysis, by the atomic absorption method and spectrophotometry. The serum concentrations of zinc and copper were found to decrease with the frequency of dialysis treatments, while the serum levels of iron and aluminium increased. Referred to the time elapsed since the start of the dialysis programme, the serum concentrations of zinc decreased, while those of iron, copper and aluminium increased. Attention is drawn to published observations pointing to the hazard of aluminium intoxication and to the symptoms and consequences of copper and zinc deficiency.