Seventy-five thousand 5-day-old babies were screened for cystic fibrosis by blood spot immunoreactive trypsin (IRT) assay as part of a statewide screening program. IRT was elevated in 433 babies; retesting revealed persistent elevation in 38. Sweat testing confirmed cystic fibrosis in 35 babies and was normal in two babies, whose IRT remained elevated at the time of the test. Sweat testing was refused by one mother. Of the 35 babies with cystic fibrosis, 13 had meconium ileus or an already diagnosed affected sibling, but the diagnosis was unsuspected in 22, although all but four had some symptoms suggestive of cystic fibrosis. Stool trypsin activity at the time of the diagnostic screen was normal in nine and reduced in seven of the babies with cystic fibrosis. One baby did not have elevated IRT, and the cystic fibrosis was missed by the screening test. In a retrospective study of blood spot samples from 36 newborn infants, who were later diagnosed as having cystic fibrosis, all had IRT levels greater than in matched controls. Our study confirms that elevated IRT is characteristic of newborn babies with cystic fibrosis, and shows that this test is very specific and sensitive when used as a newborn screening test.