A case of hypomagnesaemia due to a urinary leak of magnesium probably due to distal tubular damage by cytotoxic drugs is described. Tetany occurred 2 days after hysterectomy for choriocarcinoma with a plasma calcium of 1.54 mmol/l and magnesium of 0.15 mmol/l. The tetany was abolished, plasma calcium restored to normal and hypomagnesaemia improved by magnesium therapy alone. Circulating parathyroid hormone levels were low suggesting that the hypocalcaemia resulted from hypoparathyroidism induced by the initial severe hypomagnesaemia.