Ten patients with remitted bipolar illness on lithium maintenance therapy underwent placebo-controlled lithium discontinuation. Clinical ratings and recording of sleep-wake activity using wrist-worn actigraphs were carried out before and after lithium discontinuation. Seven patients experienced relapse into mania or hypomania within the first 3 months after lithium discontinuation. Actigraphic recordings revealed that patients who relapsed had higher baseline levels of daytime motor activity than patients without relapse. This may suggest that motor activity can be a sensitive marker of subclinical manic tendencies and early relapse following lithium discontinuation.