Although hypomagnesemia is common and causes weakness in humans, it is not known what effect its correction has on respiratory muscle power. We designed an experiment to test the hypothesis that correction of hypomagnesemia will improve respiratory muscle power. We measured muscle power and lung volumes in 16 control subjects and 17 patients (11 alcoholics, 6 nonalcoholics) on 3 consecutive days. After baseline measurements on Day 1, patients were randomized and the study was double blinded. Patients were given magnesium intravenously or dextrose as placebo. Measurements were repeated on Day 2, and the patients were then given magnesium if they had previously been given placebo, and vice versa. Measurements were done again on Day 3. There was no significant difference in lung volumes in patients or control subjects over the 3 days. There were no changes in respiratory muscle power in control subjects, but there was an improvement in all parameters of muscle power measured after magnesium therapy in patients. Hypomagnesemia should be excluded as a cause of muscle weakness because its replacement may improve muscle power.