A polymorphic trinucleotide repeat (CGG/GCC) within the human Reelin gene (RELN) was examined as a candidate gene for autism spectrum disorders (ASDs). This gene encodes a large extracellular matrix protein that orchestrates neuronal positioning during corticogenesis. The CGG-repeat within the 5Ј untranslated region of RELN exon 1 was examined in 126 multiple-incidence families. The number of CGG repeats varied from three to 16 in affected individuals and controls, with no expansion or contraction observed during maternal (n ‫؍‬ 291) or paternal (n ‫؍‬ 287) transmissions in families with autistic probands. Although the frequencies of the RELN alleles and genotypes in affected children were not different from those in the comparison group, a family-based association test (FBAT) showed that the larger RELN alleles (Ն11 repeats) were transmitted more often than expected to affected children (S ‫؍‬ 43, E(S) ‫؍‬ 34.5, P ‫؍‬ 0.035); this was particularly the case for the 13-repeat RELN allele (S ‫؍‬ 22, E(S) ‫؍‬ 16, P ‫؍‬ 0.034). Affected sib-pair (ASP) analysis found no evidence of excess sharing of RELN alleles in affected siblings. The impact of genotypes with large alleles (Ն11 repeats) on the phenotypes in individuals with ASD was analyzed by ANOVA in a subset of the families for which results of the Autism Diagnostic Interview-Revised were available. Children with large RELN alleles did not show any difference in scores for questions related to the core symptoms of autistic disorder, but there was a tendency for children with at least one large RELN allele to have an earlier age at first phrase ( 2 ‫؍‬ 3.538, P ‫؍‬ 0.06). Thus, although the case-control and affected sib-pair findings did not support a role for RELN in susceptibility to ASD, the more powerful family-based association study demonstrated that RELN alleles with larger numbers of CGG repeats may play a role in the etiology of some cases of ASD, especially in children without delayed phrase speech.