Cystic Fibrosis (CF) is the most common lethal inherited disease among U.S. Caucasians. Its effects on the respiratory system, characterized by recurrent acute bronchopulmonary exacerbations on top of ongoing chronic infection of the airways, leads to progressive inflammation, bronchiectasis, respiratory insufficiency, and death. Treatment of acute exacerbations of pulmonary symptoms has been a mainstay of CF care and management, commonly involving 14-21 day courses of an aminoglycoside and an antipseudomonal beta-lactam antibiotic targeted against Pseudomonas aeruginosa. The optimal dosing schedule for aminoglycosides, such as tobramycin, has been debated; but there is evidence that once-daily dosing may be more effective and less toxic than conventional q8-hourly dosing. This study compares the safety and efficacy of these two dosing schedules (once-daily vs. q8-hourly) in CF patients presenting with a Pseudomonas aeruginosa pulmonary exacerbation requiring IV tobramycin. Certainly, there would be savings in terms of time, effort and health care dollars if once-daily dosing is at least as safe and as effective as q8-hourly dosing. This is a double-blind, randomized cross-over study design and the "code" will be broken at the completion of the study.