47 127 newborn infants from the Emilia-Romagna region were screened for cystic fibrosis by dried blood spot trypsin assay. In the initial 12 099 subjects screened with a non-standardized method, two children with cystic fibrosis were observed. Of the remaining 35 028 newborn infants, 299 showed high immunoreactive trypsin values; retesting revealed persistent elevation in 11. Sweat testing confirmed cystic fibrosis in 6 subjects and was normal in 5. Clinical monitoring of these 5 children has, as yet, shown no pathological signs. No false-negative test results have yet been identified. In our region, cystic fibrosis frequency would appear to be 1 case every 5 890 newborn infants. Our study confirms that elevated immunoreactive trypsin is characteristic of newborn infants with cystic fibrosis and that screening by determination of immunoreactive trypsin is of great benefit since it allows early diagnosis and a rational approach to therapy.