Intermediate-length cytosine-adenine-guanine nucleotide repeat expansions in the ATXN2 gene (which encodes for the protein Ataxin-2) have been linked to increased risk for amyotrophic lateral sclerosis (ALS) in different populations. There is no such study in the Brazilian population, which has a mixed ethnic background. We have thus selected 459 patients with ALS (372 Sporadic ALS and 87 Familial ALS) and 468 control subjects from 6 Brazilian centers to investigate this point. We performed polymerase chain reaction to determine the length of the ATXN2 alleles. Polymerase chain reaction products were resolved using capillary electrophoresis on ABI 3500 × l capillary sequencer. We found that ATXN2 intermediate-length expansions (larger than 26 repeats) were associated with an increased risk for ALS (odds ratio = 2.56, 95% confidence interval: 1.29–5.08, p = 0.005). Phenotype in patients with and without ATXN2 expansions was similar. Our findings support the hypothesis that ATXN2 plays an important role in the pathogenesis of ALS also in the Brazilian population.