The incidence of acute renal failure, hypertension and electrolyte disorders in 64 bone marrow transplant recipients randomized to receive either cyclosporin or cyclophosphamide was investigated. Sixty-four per cent of patients developed acute renal failure, 75 per cent hypertension, and 88 per cent significant hypomagnesemia. The incidence of diastolic hypertension and hypomagnesemia was greater in the patents treated with cyclosporin. Hypomagnesemia was due to magnesium wasting by the kidney. Both groups received similar cumulative doses of aminoglycoside antibiotics. Significant proteinuria developed in all but one patient and nephrotic-range proteinuria was noted in 21 per cent. The cause of the proteinuria is unclear; no obvious morphologic changes were seen at autopsy in patients who exhibited nephrotic-range proteinuria. The abnormalities of renal function were shown to be transient in patients who were observed for periods ranging from one to three years. It is concluded that hypertension, renal failure and hypomagnesemia are common in the setting of bone marrow transplantation. Whereas cyclosporin probably aggravates the severity of these disorders, it is likely that other factors (e.g., aminoglycoside antibiotics) play a major role as shown by the high incidence of renal and electrolyte disorders in patients treated with cyclophosphamide alone.