This study analyzed the increase in head circumference (HC) of 85 autistic infants (64 boys and 21 girls) during their first year of life. The data were collected from their "mother-and-baby" notebooks. This notebook is a medical record of the baby's growth and development delivered to the parents of all babies born in Japan. This is a retrospective study which gathered the data from the notebooks after the diagnosis of autism. However, none of the babies were known to have autism at the time the records were made. The head circumference at birth of these autistic children was similar to that of the average found in a Japanese Government Study of 14,115 children. However, it showed a marked increase at 1 month after birth. The discrepancy reached a peak at 6 months, while the difference became smaller at 12 months. Body length (BL) and body weight (BW) began to increase at 3 months, although at a rate smaller than the head circumference increase.