Among 436 959 live infants born between 1970 and 1975 in Denmark were 72 (49 girls and 23 boys) who developed primary hypothyroidism, giving an incidence of 1 in 6064 live births. In the oldest group (6 to 8 years of age) the incidence was 1 in 5100. The age at diagnosis ranged from 1 week to 5 years 4 months; 10% were diagnosed within the first month, 40% within the first 3 months, and 70% within the first year of life. Thyroid scintigrams were available for 43 patients, 72% had aplastic or hypoplastic thyroid glands, 12% had ectopic thyroid glands, 16% had goitres. Patients with aplastic or hypoplastic thyroids tended to be diagnosed earlier than those with ectopic thyroid glands. The calculated incidence in the presence study is lower than that suggested by the Danish neonatal screening programme.