In symptomatic patients with cystic fibrosis, the recovery of bacteria in an inflammatory exudate from the lower respiratory tract is strong evidence of endobronchitis. It is not known when this chronic infection begins, the etiologic agents during infancy or the mechanism of evolution from Haemophilus influenzae and Staphylococcus aureus to Pseudomonas aeruginosa. Antibiotic administration to "suppress" the infection in relatively well patients is an unproven benefit. During an exacerbation of bronchitis, administration of appropriate antibiotics decreases sputum bacterial density and is accompanied by decreased amounts of indicators of inflammation in sputum: pulmonary function improves, particularly that reflecting medium to small airway status. In the future aggressive diagnostic procedures will be followed by therapeutic and prophylactic antibiotic administration conducted in a manner to minimize emergence of antibiotic-resistant bacteria. Adjunctive therapy, to minimize those aspects of the host response which inflict lung damage, will become standard.