Cisplatin is a chemotherapeutic agent with a high emetic potential; it can lead to hypomagnesemia, which is caused by a renal wasting. Because of beneficial effects of magnesium in the therapy of motor disorders of the upper gastrointestinal tract and its sedative, anticonvulsive effect in eclampsia, we tested parenteral magnesium sulfate as a potential antiemetic in patients receiving cisplatin. A prospective, randomized, double-blind, cross-over study was carried out in 20 patients receiving greater than or equal to 60 mg/m2 cisplatin. A standard antiemetic regimen consisting of a single dose of 10 mg diazepam and five times 0.5 mg/kg metoclopramide (every 2 h) was used. Simultaneously, 8 g magnesium sulfate or isotonic sodium chloride was infused over 4.5 h. The efficacy of magnesium was analyzed with an emetic score system; no difference was found between magnesium sulfate and sodium chloride. Only two patients were hypomagnesemic, and they had a better emetic score with the magnesium infusion. We conclude that the i.v. administration of magnesium during cisplatin therapy has no antiemetic effect, at least in normomagnesemic patients.