Serum magnesium in 79 patients of gynecologic neoplasms treated with cisplatin and their controls was measured. The results showed: (1) the average value of serum magnesium in patients following chemotherapy with cisplatin was significantly lower than in the controls; (2) the incidence of hypomagnesemia was positively correlated with the number of the courses and the total dosages of chemotherapy, being 52.9% after one to three courses of treatment and 92.0% after more than six courses of treatment; (3) the incidence of hypomagnesemia was directly related with the severity of gastrointestinal disorders; (4) serum magnesium following cisplatin chemotherapy was not correlated with serum blood urea nitrogen (BUN). Hypomagnesemia is clinically characterized by symptoms of the nervous system which are found in 14.0% of hypomagnesemic patients.