The frequency and prevalence of hypomagnesemia (less than 0.72 mmol/L) in two divisions of a consolidated medical center with two distinct patient populations, acute and chronic, was studied. In the acute care patient population, the frequency and prevalence of hypomagnesemia was 41.4% (222 of 536 determinations) and 26.1% (92 of 353 patients) compared to a frequency of 12.5% (50 of 399 determinations) and prevalence of 3.5% (9 of 258 patients) in the chronic care population. Although the two divisions have similar numbers of hospital discharges, the acute care facility handles acute medical and all surgical cases, whereas the chronic care facility handles primarily psychiatric cases. In the acute care facility, the most common diagnoses associated with hypomagnesemia were coronary artery disease, malignancy, coronary artery bypass surgery, chronic obstructive pulmonary disease and alcoholism, whereas in the chronic care facility, alcoholism, liver disease, and carcinoma were the most frequent diagnoses associated with hypomagnesemia. The frequency and prevalence of hypomagnesemia in a patient population depends on the type of patient population studied and is significantly greater in the acute care compared to the chronic care patient population.