We observed prospectively 20 consecutive patients with severe hypomagnesemia (serum Mg 0.01 to 1.2 mg/dl [0.01 to 0.5 mmol/L], mean 0.8 mg/dl [0.33 mmol/L]) before and after correction with parenteral magnesium sulfate. Only three patients, all with hypocalcemia, had tremor and muscle twitching and none showed tetany, a positive Trousseau's test, arrhythmias, or ECG abnormalities. Moreover, review of the literature on hypomagnesemia did not justify attributing these clinical symptoms to hypomagnesemia. In a follow-up study of 111 consecutive serum samples from hypocalcemic patients, 36 (32%) indicated hypomagnesemia (serum Mg no greater than 1.5 mg/dl [0.6 mmol/L]); however, hypomagnesemia had been unsuspected in all but two patients. We conclude that hypomagnesemia rarely shows specific signs or symptoms; its diagnosis depends on a high index of suspicion in patients with hypokalemia, especially after its correction, and in patients with unexplained hypocalcemia.