This study's primary goals are to test the safety and effectiveness of Tobramycin for Inhalation (TOBIr) in cystic fibrosis (CF) patients who are between 6 months and 6 years of age. This drug is an antibiotic that is inhaled into the lungs by the patient. It has already been studied and approved by the FDA for treatment of CF patients 6 years and older. Lung fluid will be examined for bacteria before and after the 28-day treatment. The amount of bacteria before and after treatment will be compared. This will indicate whether the antibiotic was effective in killing bacteria in the lungs. Once treatment begins, patients will be monitored every 2 weeks throughout the study (5 exams in 56 days). Half of the patients will receive TOBIr, half will receive a placebo (a substance that looks like TOBIr but contains no medication).

Inclusion Criteria:

• Age at least 6 months and less than 6 years
• Diagnosis of cystic fibrosis with 2 clinical features consistent with CF and confirmed by either sweat chloride >= 60 mEq/L (by quantitative pilocarpine iontophoresis) or by genotype with 2 identifiable mutations consistent with CF.
• One throat or sputum microbiology culture positive for Pseudomonas aeruginosa (Pa) within 2 weeks to 12 months prior to screening.
• Informed consent by parent or legal guardian.

Exclusion Criteria:

• History of adverse reaction to anesthesia or sedation.
• History of aminoglycoside hypersensitivity.
• History of unresolved anemia (hematocrit < 30%) or thrombocytopenia (platelet count < 100,000/mm3).
• History of hemoptysis with 30 days prior to screening.
• History of abnormal renal function (serum creatinine > 1.5 times the upper limit of normal for age).
• History of clinically documented chronic hearing loss.
• Administration of any investigational drug within 30 days prior to screening.