Psychiatric patients with tardive dyskinesia (TD) may be difficult to distinguish from those with Huntington's Disease (HD), who frequently have psychiatric symptoms. This study compared 14 patients with HD with 21 patients (15 schizophrenics and 6 with affective disorder), matched for involuntary movements, using a quantitated neurological examination and other objective and semi-objective tests. The HD group was significantly more impaired on measures of voluntary movement and saccadic eye movements. When the psychiatric group was subdivided, voluntary motor impairment was most marked in schizophrenics with both TD and parkinsonism. The implications of these findings are discussed.