A prospective analysis of physical growth in 56 children with congenital hypothyroidism compared the children's height, weight, and head circumference with population percentiles. Two measures of skeletal maturation and predictions of adult height were also compared with population standards. The mean percentiles for the children's height and head circumference were higher than population means. Although mean bone age scores were slightly higher, predictions of adult height did not differ significantly from midparental height (arithmetic mean of sum of parental heights) and population means. The duration of intrauterine hypothyroidism as measured by gestational bone age and the duration of postnatal hypothyroidism were inversely correlated with heights attained up to 9 years. This association suggests a possible long-term influence of early hypothyroidism on growth. In children with congenital hypothyroidism, maintenance of serum thyroxine levels in the upper half of the normal range results in normal growth patterns.