Gilles de la Tourette syndrome (GTS) is a chronic neurological disorder manifested by involuntary motor tics and vocalizations. Many GTS patients also suffer from obsessions and compulsions. The clinical similarities between GTS and obsessive-compulsive disorder (OCD), their occurrence among members of the same families, and the fact that both can be observed as symptoms of known basal ganglia disturbances suggest that GTS and OCD share common neurological mechanisms. It is hypothesized that the tics and vocalizations of GTS are aberrant manifestations of simple motor programs that are spontaneously generated by the basal ganglia and that obsessions and compulsions represent more complex motor plans initiated by similar anomalous activities.