In this study of 266 living patients with cystic fibrosis, ranging in age from 1 1/2 to 35 years, respiratory allergy was documented in 44 patients, or 16.6 per cent. Although a number of patients had been treated for respiratory allergy prior to the diagnosis of cystic fibrosis, more than half of those patients did not meet the requirements for a diagnosis of allergy. This study included detailed histories, skin tests, sweat tests, x-rays, and repeated clinical evaluation. The sweat-electrolyte level was markedly elevated in patients with cystic fibrosis and was well within the normal range in patients with respiratory allergy alone. It is suggested that cystic fibrosis be considered in the differential diagnosis when dealing with recurrent respiratory tract involvement in patients at all ages; one should be aware of the possible coexistence of allergy in patients with cystic fibrosis.