Thirty-seven patients with seasonal affective disorder (SAD) and 25 control subjects were given a concurrent task, the verbal-manual interference paradigm, as a measure of hemispheric language lateralization. In winter, depressed patients showed a symmetrical interference pattern and control subjects experienced greater overall disruption in right-hand tapping during concurrent verbalization. Patients showed a greater left-hand tapping decrement compared with control subjects. Light treatment decreased the decrement in tapping with the left but not with the right hand in patients. After treatment, patients did not differ from controls in dual task interference effects. Patients also showed no difference from controls in summer. The difference between depressed subjects and controls in performance measures of laterality suggests that winter depression is associated with a shift of laterality from the left to the right. The winter abnormalities appear to be normalized by bright light.