A technique using neutron activation analysis has been developed for the early diagnosis of cystic fibrosis in children and infants. Toe- and finger-nail clippings from infants (under 1 month in age) and children (under 12 years of age) are collected in a prescribed manner and irradiated in a reactor. The irradiated samples are analyzed for Na, K, and Cl concentrations, and the patients having abnormally high values of Na and Cl are detected by computer sorting.