We estimated the relative contributions of known ataxia genes (SCA1, 2, 3, 6, 7 and X25) in the patient population sent to our DNA diagnostic laboratory for diagnostic testing. Approximately 28% of these patients had an abnormal triplet repeat expansion in one of these ataxia genes (3% for SCA1, 8% for SCA2, 11% for SCA3/MJD, 2% for SCA6, 3% for SCA7, and 1.5% for X25). The lack of abnormal repeat expansions in the majority of ataxis patients tested suggests that the molecular defects associated with most ataxia cases are currently undetermined and that this population includes both familial and sporadic cases. In contrast, of the patients submitted for genetic testing for Friedrich's ataxia (FRDA), 44% (69/157) showed at least one expansion in the X25 gene, indicating that FRDA accounts for a significant proportion of the recessively inherited ataxias and appears to have a high rate of accurate clinical diagnosis. On the basis of our DNA studies, we propose a comprehensive and efficient approach for molecular analysis of ataxia patients.