Two patients with Huntington's disease (HD) and obsessive-compulsive disorder (OCD) are reported. The OCD was manifested by repetitive, stereotyped, complex, egodystonic behaviors that were disabling. These cases and other neurological syndromes with OCD (Gilles de la Tourette syndrome, neuroacanthocytosis, postencephalitic parkinsonism, caudate infarction, carbon monoxide poisoning, manganese intoxication, anoxia, progressive supranuclear palsy, Sydenham's chorea, and frontal lobe lesions) indicate that the frontal lobe, caudate nucleus, and globus pallidus are members of a complex circuit that plays a key role in mediating the symptoms of OCD. Evidence of excitatory subcortical output to cortex is shared by many neurological disorders manifesting OCD.