We measured motor activity with a self-contained monitoring device worn on the wrists of affectively ill patients and volunteer normal control subjects. Decreases in the daytime motor activity level were observed in depressed patients, compared with their improved (euthymic) or manic mood states. Moreover, affectively ill patients, even during euthymic periods, showed lower daytime motor activity levels than the control group housed in the same ward. These data provide objective evidence for decreases in motor activity that occur concomitantly with the depressive phase of illness in patients with affective disorder, and fluctuate in patients in euthymic or manic phases.