This study reports on the prevalence of hypomagnesemia (4.5%), by the stringent criterion of less than or equal to 1.25 mEq/L, and hypokalemia (17%) in 1,000 ambulatory hypertensive patients under treatment at the VA Medical Center, Oklahoma City. The hypomagnesemic group required a greater number of antihypertensive medications than the nonhypomagnesemic patients to maintain their blood pressure in the acceptable range. These observations suggest the possibility that magnesium may play an important role in blood pressure control and indicate the need for further studies.