We studied renal function and serum electrolytes in 51 patients receiving cisplatin chemotherapy by retrospectively reviewing the charts of 44 patients and prospectively following seven patients. Hypomagnesemia developed in 23 of 44 evaluable patients who were receiving cisplatin. We documented inappropriate renal magnesium wasting in four patients. Two patients required hospitalization for symptomatic hypomagnesemia. We conclude that cisplatin can induce a renal tubular defect in magnesium conservation and serious clinical syndromes of magnesium deficiency.