Filter paper blood samples collected neonatally from infants with congenital hypothyroidism were analysed retrospectively for TSH (n = 41) and thyroxine (n = 16). The patients were detected by clinical signs and symptoms and treatment was started during the first two years of life. Similar blood samples from control infants were analysed for comparison. All CH patients would have been detected neonatally if screening had been based on TSH analyses with a cut-off limit corresponding to 50 mU of TSH/l of plasma. A screening programme involving analyses of thyroxine would require considerably higher recall frequency to yield 100% sensitivity. These results support a nationwide CH screening program based on TSH analyses of PKU filter paper blood samples using 50 mU/l of plasma as the cut-off limit.