A debilitating, regularly recurring, biphasic disorder is described in 6 severely multidisabled children. It was characterized by several days of lethargy, withdrawal, loss of abilities, irritability, and hypersomnolence followed or preceded by a high-energy state for several days during which the children slept very little, at times were euphoric, had improved mental ability, and were hyperactive. These cyclic episodes had been present for years but unexpectedly disappeared in one child. The etiology is unknown, in spite of detailed neurologic, metabolic, and endocrine investigations. All patients had family histories positive for affective disorder. Melatonin treatment helped to regulate the coexisting chronic sleep disorders of 3 children but failed to eliminate the cycles. Antiepileptic drug treatment, lithium, sedatives, stimulants, tranquilizers, and light therapy were largely ineffective. The children's symptoms and signs fit the diagnostic criteria of a bipolar affective illness, as it was modified for patients with associated neurologic disability; therefore, the patients appeared to have a unique disorder that closely resembles or is a variant of rapid cycling affective disorder.