We measured serum magnesium (Mg) levels in 71 in-patients and out-patients with mood disorders and in 30 healthy controls and investigated the relationships between serum Mg levels and clinical background factors. Serum Mg levels were found to be significantly higher in patients with mood disorders than in controls. Serum Mg levels showed no significant correlation with patient sex, age, diagnosed subtype and disease phase in the mood disorder group. Serum Mg levels in patients with major depressive disorder who were taking psychotropic drugs were not significantly different from levels seen in patients with major depressive disorder who were not taking psychotropic drugs. These results suggest that the high serum Mg levels noted in patients with mood disorder are related to the underlying disorder itself and are not influenced by clinical background factors.