Two adult patients with cystic fibrosis had late onset of symptoms with principal involvement only of the lungs. The absence of other typical features of cystic fibrosis confused the diagnosis. Recovery of mucoid Pseudomonas aeruginosa from sputum cultures helped to establish the diagnosis in these patients, because these mucoid isolates are rarely obtained from subjects who do not have cystic fibrosis. Recent findings, which have identified the mucoid isolates as predominantly a single strain of P aeruginosa and have documented its widespread carriage in the United States by patients with cystic fibrosis, point to an interesting epidemiologic relationship between this bacterium and this disease.