To determine the clinical utility of thyroid ultrasonography in the diagnosis of congenital hypothyroidism (CH) before initiation of therapy, ultrasonographic images of the thyroid gland with a high-resolution transducer were obtained in 204 healthy infants aged from newborn to 12 months (Group A), and 174 infants suspected of having CH detected by neonatal mass screening (Group B). The thyroid gland was imaged by transverse scanning at the anatomic site of the thyroid gland. The maximal width of thyroid on the transverse section in the normal location was measured. By comparing with the normal thyroid gland size and location obtained from Group A, 174 infants of Group B were divided into four subgroups: 1) Normal in size (n = 117), 2) Enlarged (n = 33), 3) Small (n = 1) and 4) Invisible in the normal location (n = 23). They were compared with the final diagnoses based on the results of chemical laboratory data and scintigraphic findings. The sensitivity and the specificity for the presence or absence of the thyroid gland in the normal location were 96% (22/23) and 99% (150/151), respectively. Both subgroups of normal and enlarged sized gland included healthy infants (false positive), transient hyperthyrotropinaemia, transient hypothyroidism and CH due to dyshormonogenesis. We conclude that ultrasonography is useful for determining the presence or absence of the thyroid gland in the normal location, whereas normal and enlarged sized glands require further examination to complete the diagnosis.