Temperament was investigated in a group of high-risk infants (N = 383; 45 % girls) who had an older sibling with autism spectrum disorder (ASD), and in community control infants (N = 162; 46 % girls) with no family history of ASD (low-risk). The infants were assessed at age 12 months using the Infant Behavior Questionnaire, and at 24 months using the Toddler Behavior Assessment Questionnaire. At 36 months, an independent blind diagnostic assessment for ASD was conducted using the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (ADOS). The results indicate not only differences in temperament traits between the high- and low-risk groups, but also differences in the structure of higher-order temperament factors. The results support the importance of early reactive temperament in the development of Effortful Control in the high-risk sample. Furthermore, Effortful Control at 24 months appears to play a critical role in predicting later ASD symptoms (at 36 months). Taken together, these findings support the use of early temperament as an endophenotype for ASD.