Little is known about attachment security and disorganization in children who are at genetic risk for an Autism Spectrum Disorder (ASD) prior to a possible diagnosis. The present study examined distributions of attachment security and disorganization at 15-months of age in a sample of infant siblings of older children with (ASD-sibs; n = 51) or without (COMP-sibs; n = 34) an ASD. ASD-sibs were not more or less likely to evince attachment insecurity or disorganization than COMP-sibs. However, relative to COMP-sibs, the rate of B1-B2 secure subclassifications was disproportionately larger in the ASD-sib group. Results suggest that ASD-sibs are not less likely to form secure affectional bonds with their caregivers than COMP-sibs, but may differ from COMP-sibs in their expression of attachment security.