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).