The use of the dried-blood immunoreactive-trypsin assay for the detection of cystic fibrosis in newborns has been questioned on the grounds that it may fail to identify patients with enough pancreatic function to have normal fat absorption. To investigate this possibility, we assessed pancreatic function in 78 patients identified in a neonatal screening program as having cystic fibrosis. The diagnosis of cystic fibrosis was confirmed by abnormal results on a sweat chloride test. The results of measurements of fecal fat excretion, pancreatic-stimulation tests, and estimations of the serum level of pancreatic isoamylase indicated that 29 of the 78 children (37 percent) had substantial preservation of pancreatic function. These children (median age, four years) had growth that was close to normal and comparable to growth in children with severe pancreatic insufficiency who received oral enzyme therapy. Pancreatic insufficiency subsequently developed in 6 of the 29 patients, at 3 to 36 months of age. We conclude that the serum immunoreactive-trypsin assay used in neonatal screening programs identifies patients with cystic fibrosis who have sufficient pancreatic function to have normal fat absorption and that a substantial proportion of infants identified as having cystic fibrosis are in this category.