The use of susceptibility testing to identify appropriate antimicrobial agents has long been an important element of the practice of infectious-disease medicine. In most clinical settings, the finding that a strain of bacteria is susceptible to a specific antibiotic is a reliable indicator of the effectiveness of that drug. An exception is the treatment of pulmonary infection in patients with cystic fibrosis. Once chronic Pseudomonas aeruginosa infection is established in the lungs of these patients, the bacteria are rarely, if ever, eradicated despite treatment with combinations of antimicrobial agents with demonstrated potency in vitro. Even direct administration of aerosolized antibiotics . . .