The etiology of the respiratory lesion in fibrocystic disease of the pancreas is discussed with a summary of the congenital and nutritional hypotheses. There is no morphologic evidence that a specific congenital abnormality exists in the bronchi or in their secretion. The microscopic appearance of the mucus does not resemble that of the contents of the pancreatic ducts but does resemble the mucus seen in some other clinical conditions, notably asthma. A physiologic congenital abnormality of the bronchi is possible but unproven. Evidence is presented that dietary therapy improves the growth curves of infants diagnosed and treated prior to infection. The absorption of vitamin A, and presumably of other fat-soluble specific substances, is affected by diet. Twenty-two cases of fibrocystic disease without clinical evidence of respiratory infection are presented. Twenty-one of these were first seen after penicillin became available but only 11 have received it. The majority of the 22 patients were started on dietary therapy before a cough had been present for as long as two months. This record is presented as evidence in favor of a nutritional factor in the etiology of the respiratory infection.