The growth velocity of 46 infants with congenital hypothyroidism, classified into three groups: athyreosis (n = 12), ectopic (n = 25) and eutopic gland (n = 9), was analyzed. Height, expressed as the standard deviation score (SDS), was measured at birth, at diagnosis and 2, 4, and 8 weeks after initiation of therapy. Bone age was retarded at birth and at diagnosis, epiphyseal surfaces of the knee were correlated with serum T4 values. By contrast, height (mean +/- SD) at birth was normal (0.3 +/- 0.8 SDS) with identical distribution among the three subgroups. Growth velocity was decreased during the first weeks of life: height at diagnosis was -0.1 +/- 0.6 SDS, a value significantly different from size at birth (p less than 0.02). After therapy was started, a continuous decline in growth velocity was observed, reaching -0.2 +/- 0.7 and -0.2 +/- 0.6 SDS, respectively, at 2 and 4 weeks of therapy (p less than 0.01 vs. size at birth). Growth retardation was correlated with the degree of thyroid deficiency. Mean height after 2 weeks of therapy was at -0.6, -0.1, 0.1 SDS in patients with athyreosis, ectopic and eutopic gland, respectively. Catch-up growth occurred thereafter, and the mean height reached 0.3 SDS at 12 weeks for the entire group. A correlation between the decrease in growth velocity and T4 levels at diagnosis was found (p less than 0.05). The normal size at birth and the decreased growth rate observed at diagnosis seem to indicate that growth becomes thyroid hormone dependent immediately after birth.