The purpose of this study was to examine patterns of anxiety among siblings of children with autism spectrum disorders (ASD), and determine the characteristics of the child with ASD and their parents that predicted anxiety. Data was collected from 1,755 siblings of children with ASD whose families participated in the Simons Simplex Collection; siblings ranged in age from 3 to 18 years (M = 9 years). Male siblings were at increased risk for sub-clinical anxiety problems during middle childhood. Parental history of anxiety disorders, higher maternal pragmatic language, and more proband behavior problems predicted higher anxiety. While siblings overall did not show elevated anxiety symptoms, higher rates of sub-clinical anxiety problems among males and siblings in middle childhood are cause for concern.