Objective: To assess the frequency of SCA1 (spinocerebellar ataxia type 1), SCA2, SCA3/MJD (spinocerebellar ataxia type 3/Machado-Joseph disease), SCA6, SCA7, and DRPLA (dentatorubropallidoluysian atrophy) CAG trinucleotide repeat expansions [(CAG)n] among persons diagnosed with hereditary SCA from Chinese families. Patients and Methods: Spinocerebellar ataxia type 1, SCA2, SCA3/MJD, SCA6, SCA7, and DRPLA (CAG)n mutation were detected with the polymerase chain reaction, highly denaturing polyacrylamide gel electrophoresis, and silver staining technique in 167 patients with autosomal dominant SCA from 85 Chinese families and 37 patients with sporadic SCA. Results: Spinocerebellar ataxia type 1 (CAG)n mutation in 7 patients from 4 kindreds (4.70%) was expanded to 53 to 62 repeats. Spinocerebellar ataxia type 2 (CAG)n mutation in 12 patients from 5 kindreds (5.88%) was expanded to 42 to 47 repeats. Spinocerebellar ataxia type 3/Machado-Joseph disease (CAG)n mutation in 83 patients from 41 kindreds (48.23%) was expanded to 68 to 83 repeats. Sixty-five patients from 35 kindreds (41.19%) and 37 patients with sporadic SCA did not test positive for SCA1, SCA2, SCA3/MJD, SCA6, SCA7, or DRPLA. There was a predictable inverse relationship between the number of CAG repeats and the age at onset for SCA3/MJD and SCA2. Clinically, dementia and hyporeflexia were more frequent in patients with SCA2, while spasticity, hyperreflexia, and Babinski signs were more frequent in patients with SCA3/ MJD, and those might be helpful in clinical work to primarily distinguish patients with SCA3/MJD and SCA2 from others with different types of SCA. Conclusions: The frequency of SCA3/MJD is substantially higher than that of SCA1 and SCA2 in patients with autosomal dominant SCA from Chinese kindreds, who are non-Portuguese. Clinical expressions of the various types of SCAs overlap one another; therefore, for clinical study it is important to make a gene diagnosis and genetic classification for patients with SCA.