We studied the factors that might influence onset age in Friedreich's ataxia in 41 cases (20 male, 21 female) homozygous for GAA expansion on the first intron of X25 gene. Patients came from 18 multiplex families (13 couples, 5 triplets). Mean age (SD) was 18.1 (8.9) years and did not differ by gender. Onset age and the sizes of the smaller (GAA1) and the larger (GAA2) allele in each pair showed high intrafamily correlation. We found an inverse correlation between age at onset and GAA1 size, but not between age at onset and GAA2 size. Stepwise multiple regression of onset age on GAA1 size, sibling onset age, and GAA2 size showed that GAA1 accounts for 73% of onset age variance, and sibling onset age for an additional 13%. The study demonstrates that, in addition to GAA expansion size, other environmental or genetic familial factors influence disease expression.