The study will include 10 adult patients with cystic fibrosis. The aim of the study is to evaluate whether Tobramycin 300 mg aerosol once-a-day for 28 days can reduce the bacterial load in the airways. Secondary end-point is the evaluation of variation in bacterial susceptibility during and at the end of study period.

Ten adult patients with cystic fibrosis will be enrolled in a stable phase and will received prophylaxis with aerosol of Tobramycin 300 mg once a day for 28 days.

Inclusion Criteria:

• Outpatients, male and female, age range 18-45 years
• Diagnosis of cystic fibrosis
• FEV1 >50% predicted.
• sputum samples available
• Chest x ray negative for pneumonia and tuberculosis
• Informed consent

Exclusion Criteria:

• Allergy to tobramycin
• Use of systemic steroids in the previous 2 weeks
• Pregnancy or breast feeding
• Treatment with other experimental drug in the previous 3 months