Cystic Fibrosis is an autosomal recessive disorder characterized by chronic pulmonary bacterial colonization and progressive lung disease. By the second decade of life, Pseudomonas aeruginosa is the primary respiratory pathogen. This study is an open label trial that uses aerosol administration of tobramycin solution to deliver high concentrations of the drug directly to the site of infection. Patients will recieve 300 mg/5mL tobramycin solution for inhalation twice daily for 28 days, followed by 28 days off treatment.