Hypomagnesemia is a well-recognised complication of cis-diamminedichloroplatinum (DDP) treatment. We prospectively evaluated 50 patients with advanced malignant disease receiving DDP for the development of hypomagnesemia. Urinary magnesium excretion was measured in 24 patients. The mean serum magnesium fell from 0.79 mmol/l (normal 0.7-1.1 mmol/l) prior to therapy to 0.55 mmol/l 3 months after commencing DDP. All 50 patients had become hypomagnesemic by this time and 10% were symptomatic, requiring oral magnesium supplementation. At 6 weeks after commencing DDP only four patients had restricted urinary magnesium excretion to less than 1.0 mmol/day. The other patients clearly had inappropriately high levels of urinary magnesium excretion, suggesting that DDP may induce a renal tubular defect in magnesium conservation. Hypomagnesemia is a common complication of DDP therapy which in many patients is asymptomatic. Further, more detailed studies of renal magnesium handling are necessary to determine fully the effect of DDP on urinary magnesium excretion.