Thyroid hemiagenesis is a rare congenital anomaly in which one lobe of thyroid gland fails to develop. Because variations of the prevalence of this anomaly have been reported, the aim of this study was to evaluate the prevalence rate of thyroid hemiagenesis in an apparently normal population from Northern Poland. Ultrasound examination of the thyroid gland was performed in 4004 unselected 7-15-year-old school-children from the seaside zone of Northern Poland. Two cases of thyroid hemiagenesis were found, both being absence of the left lobe in two girls. Thyroid volumes, adjusted to body surface area, were within normal range; serum thyrotropin, free thyroxine, and free triiodothyronine were within normal limits. Physical examination, abdominal ultrasound, and echocardiography did not show extrathyroidal malformations. Thyroid ultrasound was normal in the girls, parents, and siblings. The study showed a 0.05% prevalence of thyroid hemiagenesis in asymptomatic schoolchildren population from iodine-sufficient area of Northern Poland.