SCA1 is a dominant spinocerebellar ataxia (SCA) and a multi‐systemic syndrome caused by abnormal expansion of unstable CAG repeat in a novel gene located on chromosome 6p22–p23. We clinically studied 35 Japanese SCA1 patients who were assumed to have come from a common origin. The age at onset ranged from 15–63 years, and significantly correlated with CAG repeat units of mutant alleles. Ataxia was the initial symptom, and the majority of patients had a similar history of signs and symptoms. Nystagmus was at first minimal, later attenuated, and a slow saccade followed. Limb tendon reflexes were mostly hyperactive and depressed with the development of diffuse amyotrophy. The cardinal feature was ataxia‐hyperreflexia‐late slow saccade syndrome with terminal amyotrophy. Although the phenotype of SCA1 overlaps with those of other dominant SCAs, some facets of the neurological events differ from either SCA2 with ataxia‐hyporeflexia‐slow saccade syndrome, or early‐onset Machado‐Joseph disease with dystonia‐bradykinesia‐spasticity syndrome.