The concentrations of calcium, sodium, potassium and magnesium in the erythrocytes of patients were measured in the active and remission phases of depressive disorders. Twelve patients in the active phase and 19 patients in remission with major depression were studied and compared with 20 age-matched healthy controls. Patients with major depression in both active and remission phases showed significantly lower calcium concentrations in the erythrocytes compared with controls, although no significant differences in sodium, potassium or magnesium concentrations were found among the three groups. In addition, no differences were found in the electrolyte concentrations between the active and remission phases in the same patients. This calcium concentration had no relationship to the age, gender, or medication drugs of the subjects. Low calcium concentrations were found in the erythrocytes of depressed patients, which may be a relevant marker for depression.