From November 1986 to September 1987, 27,253 newborns were screened for congenital hypothyroidism (CH) with Time-Resolved Fluoroimmunoassay (TR-FIA) for TSH concentrations in specimens of dried blood spots on filter paper. Specimens were obtained in 12 hospitals by heel pricking of the babies 48-72 hours after birth. The specimens with TSH concentration higher than 20 mu u/ml were retested, and the babies were called back for confirmation of CH by TSH, T3, T4, Tg serum radioimmunoassay etc. and bone-age radiography if the repeated tests were positive. In the present series, 337 specimens were retested (1.23%). Altogether 25 babies were recalled (0.09%) and CH was confirmed in 5 cases, giving a screening incidence of 1:5 450. In addition, one case failed to respond to the recall because of a wrong address. TR-FIA or DELFIA (Pharmacia-LKB) is non-radioactive and does no harm to man and the environment, and is thus feasible in ordinary laboratories. The method is sensitive and the results are reproducible, with a precision of 1.4 mu u/ml. Being rapid in terms of measurement time, it is suitable for mass screening.