ABSTRACT.A total number of 112 children with congenital hypothyroidism were diagnosed in all Children's Hospitals and Pediatric Wards in Sweden during the 7‐year period 1969–1975. Since it may be assumed that all cases of congenital hypothyroidism, which were diagnosed during that period were seen in one of these hospitals, the incidence of congenital hypothyroidism in Sweden can be calculated to be 1:6900 live births. In spite of an efficient National Health Care Program for Infants the diagnosis was delayed until after an age of three months in 52% of the cases. This fact supports the view that mass screening of newborns for congenital hypothyroidism has to be introduced in Sweden. However, the beneficial effects of such a program cannot be fully elucidated until it has been considered whether earlier instituted treatment would have improved the outcome of children in whom a diagnosis was made after 3 months of