A total of 1,228,551 newborn babies, who were almost all of babies born in Osaka for 14 years (168 months), were screened for congenital primary hypothyroidism by an identical mass-screening program using the thyrotropin method, and 429 patients with hypothyroidism due to thyroid dysgenesis (dysgenetic hypothyroidism) were found. The occurrence of the patients in every month was not random but episodic and the incidence was higher in the late autumn (from October to December). These observations support a hypothesis that some environmental factors may cause this disorder overtime and the possibility of relation with intrauterine viral infection was discussed.