BACKGROUND: Longitudinal studies of head circumference growth in infants later diagnosed with autism are needed to understand the accelerated head growth in this disorder. METHODS: We analyzed longitudinal head circumference data from birth to 3 years in 28 children later diagnosed with autism spectrum disorder on the basis of individual growth curve analyses using hierarchical linear models. RESULTS: Head circumference Z scores relative to norms significantly increased in the autism sample from birth to 12 months, but this pattern did not persist beyond 12 months. Rather, the rate of change in head circumference from 12 to 36 months was not different from the normative sample. CONCLUSIONS: These results suggest that a period of exceptionally rapid head growth occurs during the first year of life in autism; after 12 months of age, the rate of head circumference growth decelerates relative to the rate during the first year of life. Studies of behavioral development in infants later diagnosed with autism suggest that the period of acceleration of head growth precedes and overlaps with the onset of behavioral symptoms, and the period of deceleration coincides with a period of worsening of symptoms in the second year of life.