Objective: Individuals with autism as young as 2 years have been observed to have larger brains than healthy comparison subjects. Studies using head circumference suggest that brain enlargement is a postnatal event that occurs around the latter part of the first year. To the authors'knowledge, no previous brain imaging studies have systematically examined the period prior to age 2. In this study they used magnetic resonance imaging (MRI) to measure brain volume in 6-month-olds at high familial risk for autism. Method: The Infant Brain Imaging Study (IBIS) is a longitudinal imaging study of infants at high risk for autism. This cross-sectional analysis compared brain volumes at 6 months of age in high-risk infants (N=98) and infants without family members with autism (N=36). MRI scans were also examined for radiologic abnormalities. Results: No group differences were observed for intracranial, cerebrum, cerebellum, or lateral ventricle volume or for head circumference. Conclusions: The authors did not observe significant group differences for head circumference, brain volume, or abnormalities in radiologic findings from a group of 6-month-old infants at high risk for autism. The authors are unable to conclude that these abnormalities are not present in infants who later go on to receive a diagnosis of autism; rather, abnormalities were not detected in a large group at high familial risk. Future longitudinal studies of the IBIS study group will examine whether brain volume differs in infants who go on to develop autism.