The effects of neonatal diagnostic screening on cystic-fibrosis (CF) -related morbidity were evaluated by comparing hospital admissions for CF-related illness in the first 2 years of life in 40 patients detected by means of neonatal screening and 56 patients born in the 3 years before screening began. Unscreened patients without meconium ileus had a mean of 27.25 hospital days for CF-related illness, and screened patients a mean of 3.9 days. There was no trend with time towards fewer days spent in hospital: the change was sudden. The difference was significant and could not be attributed to non-comparability of groups, changes in admission policy, or changes in management. In patients with meconium ileus there was no significant difference in hospital admissions between the groups. Neonatal screening significantly reduces CF morbidity in the first 2 years of life.