It has been reported that mental retardation due to congenital hypothyroidism can be prevented by early detection and early adequate replacement therapy. We have developed a radioimmunoassay for TSH using the dried blood spot and have started screening for newborn congenital hypothyroidism using a part of sample of the inborn metabolic error screening. (1) The dried blood spot TSH of 61,000 newborn infants was assayed in the first half of our screening and that of 74,505 newborn infants was assayed in the latter half of our screening. As a result, although we were not able to detect any cases in the first screening, we were able to detect 9 cases of congenital hypothyroidism in the latter screening. From the results obtained through our investigation of the thyroid function of these 9 infants, we confirmed that mild hypothyroidism can be better detected by the screening of TSH. (2) As to the program of the screening, we chose from the latter half of our screening all the samples in which TSH concentrations contained above 3 percent of each assay and were remeasured on the next assay. (3) As we confirmed that the sensitivity of measurement was increased at very low concentrations, when the volume of antibodies, radioisotopes and eluates used for each assay were decreased, we measured TSH successfully using two 3 mm discs. (4) As we can perform very simple screening by the 3 mm disc method, we are changing the screening method from that with 10 mm disc to one with two 3 mm discs. We intend to extend our screening, and will make every effort to prevent mental retardation due to congenital hypothyroidism.