An unusual family is presented in which asymmetric cerebellar signs, particularly intention ataxia and tremor, are associated with asymmetric and asychronous myoclonus, having onset in childhood, in the absence of other neurologic signs or symptoms. Elevation of the cerebrospinal fluid uric acid content has occurred in two of these patients. The disease may be inherited as an autosomal Mendelian dominant trait. The literature on myoclonus is reviewed and it is concluded that the relationship between myoclonus and signs of cerebellar dysfunction depends upon a common lesion in the dentate nucleus of the cerebellum. Myoclonus is regarded as secondary to irritative lesions of the dentate producing the discrete paroxysmal discharges eventually via the corticospinal motor system. It may appear as a relatively discrete syndrome, with fairly good prognosis, as in this family, or be associated with grand mal seizures and mental deterioration, as part of a diffuse neuronal disease.