Spinocerebellar ataxias (SCAs) are a heterogeneous group of neurodegenerative disorders. CAG repeat expansions in the causative genes have been identified as the basic cause of several types of SCAs, and have been used for the diagnoses and classifications of patients with ataxia. In order to assess the frequency and CAG repeat size ranges of SCAs, and to establish an effective strategy for molecular diagnosis, we performed a molecular analysis of SCA1, SCA2, SCA3, SCA6, and SCA7 in 76 patients. These patients were as follows: 32 with dominant inheritance, 39 sporadic cases, and 5 with unknown family histories. The normal and affected CAG repeat size ranges were established at five SCA loci in Koreans, which was consistent with previous reports. The total prevalence of the five types of SCAs was 39.5% in the 76 patients with ataxia, regardless of their family history. It was 75.0% in the 32 families with a dominant inheritance. The most frequent type was SCA3 (15.8%), followed by SCA2 (14.5%). Both types combined formed 76.7% of the 30 patients with CAG expansions. SCA1, SCA6, and SCA7 were less frequent, affecting 3.9%, 2.6%, and 2.6% of the cases, respectively. This mutation spectrum is quite different from a previous report concerning Koreans, but is similar to the distributions that are seen in several ethnic populations worldwide. For a correct and effective diagnosis of SCAs, we suggest that a molecular diagnosis be undertaken, even in patients without a family history, as well as those with a family history. A stepwise approach is also recommended. Patients with ataxia should be tested for SCA2 and SCA3. Individuals testing negative should be tested for SCA1, SCA6, and SCA7.