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Serial head circumference data for 102 male and 109 female participants in the Fels Longitudinal Study were analyzed. A 4-parameter mathematical model was fitted to the data from birth to 18 years for each participant. The estimated parameters were used to calculate initial size, age at peak growth velocity and the maximum velocity during pubescence. The estimates of initial size (birth) and final size were significantly greater for the boys than for the girls. Also peak growth velocity occurred later in the boys than in the girls, but the overall acceleration and the maximum rate of growth during pubescence were greater in the girls. Pubescent spurts in head circumference were compared for children bom during 1928-1937, 1955— 1960 and 1961-1967. In general, in each sex, there has been a secular trend towards the later occurrence of peak growth velocity and for the maximum velocity during pubescence to be less. Intrafamilial correlations for esti­mated values from the model were not significantly different from zero for parent-offspring pairings. All the corresponding correlations for sib-sib pairings were significantly different from zero, which may be attributed to the sharing of a common environment.