The growth of 361 children with congenital hypothyroidism diagnosed by screening was assessed by estimating mean values for height, weight, body mass index (BMI), and head circumference on each birthday up to the age of 4 years. In the group of children with severe congenital hypothyroidism (pretreatment plasma thyroxine < or = 30 nmol/l), the mean heights at 1 and 2 years were less than standards for healthy children, but this may be due to technical factors related to difficulties in measuring young infants and the method used to estimate height on each birthday. By the age of 3-4 years the values for mean height in the children with either severe or less severe congenital hypothyroidism were equal to or greater than those for healthy children. At all ages mean head circumference in boys and girls with severe congenital hypothyroidism was greater than standards for healthy children, but this only reached statistical significance in girls at 1 year. With the exception of the results for boys at 1 year, mean values for BMI were slightly greater in the children with severe congenital hypothyroidism. The mean BMI results for children with either severe or less severe congenital hypothyroidism were significantly greater than those for healthy French children at all ages, but they showed the same trends with increasing age. It is concluded that by the age of 3-4 years stature is essentially normal in children with early treated congenital hypothyroidism but that the increased head size reported before screening may still be evident in early infancy.