The authors measured intracellular Ca2+ concentrations in four manic and five bipolar depressed patients and seven comparison subjects. Platelet and lymphocyte intracellular Ca2+ concentrations were comparable. The patients' mean intracellular Ca2+ concentrations were higher than those of the comparison subjects and demonstrated more interindividual variation. These findings suggest a diffuse abnormality in mechanisms affecting intracellular calcium homeostasis in bipolar disorder.