After an experimental neonatal screening program for cystic fibrosis had been carried out in the Netherlands during 1973 to 1979, a follow-up study to evaluate the effects of neonatal screening was started in 1980. Although before 1980 the management of patients with cystic fibrosis was partly left to local hospitals, from the start of the follow-up program all patients in the study received similar treatment. A cumulative survival rate, calculated with exclusion of the patients with meconium ileus, showed at the age of 11 years a significantly better survival rate (p less than 0.05) for the 19 patients from the screened population (88%) than for the 25 patients from the nonscreened population (60%). Clinical condition was assessed on entry and at the age of 9 years in 16 screened and 20 nonscreened patients. On entry, comparison showed significantly better chest radiograph scores for the screened patients but no other significant differences. At the age of 9 years, after several years of similar treatment for all patients in the study, significantly better clinical (p less than 0.02) and chest radiograph scores (p less than 0.01), lower IgG levels (p less than 0.05), and higher vitamin A levels (p less than 0.01) were observed in the screened patients. Our study results suggest that early diagnosis and appropriate treatment may prevent serious deterioration and death at a young age, and may reduce the extent of early irreversible lung damage in patients with cystic fibrosis.