Mass newborn screening for primary hypothyroidism was introduced in Switzerland on January 1st, 1977, using a radioimmunoassay of TSH in dried blood spotted on filter paper. After incubation for 38 h at 20 degrees C, bound and free TSH is separated by double antibody precipitation. The filter paper discs of 6.5 mm diameter remain in the test tubes. At present, one TSH determination costs approx. SFr. 4.40. All reagents used are commercially available and their costs amount to not more than 15% of the total expenses. During the first 8 months of 1977, of 21862 newborns tested routinely on day 5 (together with the Guthrie-test), 7 infants with primary hypothyroidism were discovered owing to blood TSH values of greater than 100 muU/ml. Diagnosis was not recognized clinically although all of the infants showed some symptoms. Thyroxin therapy was started within the second week of life. The incidence of about 1 in 3000 newborns is higher than reported so far. It has to be shown whether this is due to genetic or geographic factors, to the occurrence of transitory forms, or to a higher efficiency of screening by the TSH (versus T4) assay.