CF patients followed by the Adult CF Program at National Jewish Health, who are ≥ 18 years and present with signs and symptoms of an acute pulmonary exacerbation. All subjects will be treated with at least two pathogen specific I.V. antibiotics for a minimum of 10 days, along with standard guidelines for other aspects of care for an acute pulmonary exacerbation. Concomitant use of inhaled antibiotics and systemic steroids will be allowed, and typical co-infections or co-morbidities will not result in exclusion.