Machado-Joseph disease (MJD) is a form of dominantly-inherited ataxia originally described in people of Azorean and Portuguese descent. The disorder has subsequently been identified in Japan, Brazil, Australia, and China. Average age of onset is 35 to 40. Core features include progressive ataxia, dysarthria, postural instability, nystagmus, eyelid retraction, and facial fasciculations. Dystonia is often prominent in younger patients. Three distinct phenotypes appear to reflect the clinical spectrum of a single mutant gene. Neuropathology involves afferent and efferent cerebellar systems, with preservation of cerebellar cortex and inferior olive. Spinocerebellar pathways, substantia nigra, and cranial nerve motor nuclei are involved. The disorder is due to an unstable CAG repeat on chromosome 14q32.1. A dominantly inherited ataxia (SCA-3) in families of French and German descent has also been linked to this segment of chromosome 14. The relationship between MJD and the other dominant inherited ataxias is discussed.