A patient with disseminated ovarian carcinoma presented with symptoms of hypocalcaemia secondary to hypomagnesaemia. The low serum magnesium (0.4 mmol/l) appeared to be due to renal leakage with no evidence of ureteric obstruction or hydronephrosis on intravenous urogram. Parathyroid hormone activity (serum levels and cAMP response) was normal, despite hypomagnesaemia. The patient's complaints resolved after magnesium and calcium supplementation. Hypomagnesaemia of malignancy is a rare but important complication, and is both poorly recognized and understood.