The OPCAs are not, as Dejerine and Thomas (6) and subsequent writers have suggested, a unitary morbid entity. They comprise a heterogeneous group of disorders sharing certain major clinical and morphologic features with the hereditary and acquired ataxias and with the extrapyramidal system disorders, such as Parkinson's disease and the hereditary and acquired choreas, yet distinct from both these general groups of neurodegenerative diseases. Some of the OPCAs are sharply defined morbid entities of known etiology identified on combined genetic and clinical grounds or by a unique biochemical defect; most are of uncertain provenance and may themselves represent a collection of multiple, as yet undetermined entities. The nosologic concept of OPCA survives today in part because of its (Formula: see text) clinical usefulness in categorizing patients with mixed cerebellar and extrapyramidal syndromes and its importance in the differential diagnosis of both extrapyramidal and cerebellar disorders.