Inclusion Criteria:

• Patients of either sex aged ≥ 6;
• Clinical diagnosis of cystic fibrosis defined as: (1)Patients preferably registered in the National Registry of CF (or other documents depending on country legislation); (2) Evidence of two or more typical pulmonary clinical features observed in CF, e.g., persistent colonization/infection with typical CF pathogens, chronic cough and sputum production, persistent chest radiography abnormalities, airway obstruction, nasal polyps and/or digital clubbing;
• Positive response in the standard sweat test (sweat chloride concentration ≥ 60 mmol/l for the standard method or ≥ 80 mmol/L for a microduct technique) documented in the clinical records and/or gene mutation documented in the clinical records;
• Chronic colonization of P. aeruginosa: presence in a sputum or throat culture of a minimum of 2 positive samples for P. aeruginosa over the previous 12 months and/or presence of more than two precipitating antibodies against P. aeruginosa;
• Sputum containing P. aeruginosa susceptible to tobramycin (defined as a zone diameter ≥ 16 mm after testing with 10 µg tobramycin disk or as a minimal inhibition concentration based on microdilution testing system) as identified by local laboratory at screening visit;
• FEV₁ ≥ 40% and ≤ 80% of the predicted normal value;
• Written informed consent obtained by parents/legal representative according to local regulations)and by the patient (when appropriate).

Exclusion Criteria:

• Administration of antipseudomonal antibiotic therapy by any route in the previous 4 weeks;
• Evidence of impaired renal function (serum creatinine level ≥ 1.5 mg/dl);
• Evidence of impaired auditory function (auditory threshold in either ear above 20 dB at frequencies between 250 and 8000Hz);
• Sputum culture containing Burkholderia cepacia;
• Patients with end-stage lung disease, candidates for heart-lung transplantation;
• History of other clinically significant cardiac, renal, neurological, gastrointestinal, hepatic or endocrine disease related to cystic fibrosis, whose sequelae and/or treatment can interfere with the results of the present study;
• Female subjects: pregnant or with active desire to be pregnant, lactating mother or lack of efficient contraception in a subject with child-bearing potential (i.e., contraceptive methods other than rod containing a hormone that prevents user from getting pregnant and that will be placed under the skin, syringes that contain a contraceptive hormone, combined birth control pill, i.e., such that contains two hormones, some IUDs and sexual abstinence). A pregnancy test in urine is to be carried out in women of a fertile age at screening and at the last clinic visit;
• Known hypersensitivity to aminoglycosides;
• Patients with evidence of alcohol or drug abuse, likely to be not compliant with the study protocol or likely to be not compliant with the study treatments;
• Participation in another clinical trial with an investigational drug in the four weeks preceding the screening visit (Visit 1).