The purpose of this study is to determine whether the nasal inhalation of Colistin is effective to decrease the Pseudomonas aeruginosa bacterial count in the nasal lavage fluid.

Inclusion Criteria:

• subject has a confirmed diagnosis of cystic fibrosis
• detection of P. aeruginosa in nasal lavage (culture) with chronic P.a. colonization of the lung (from day -28)
• informed consent of the patients or parents
• subject >= 6 years
• subject is able to comply with the inhalation procedures and nasal lavage procedures scheduled in the protocol
• women of childbearing potential are only included into the study, if they are using an effective method of birth control during the protocol

Exclusion Criteria:

• subject has a critical condition defined as: forced expiratory volume at one second < 30% and / or arterial oxygen saturation < 93% without O2-substitution; need of O2-substitution
• subject had an ear, nose, and throat surgery within 3 months prior to study
• subject shows signs of nasal bleeding
• subject has an ear drum perforation
• subject had an acute rhinosinusitis or a pulmonary exacerbation at study entry with need of additional systemic antibiotic therapy against pseudomonas aeruginosa
• subject is unlikely to comply with the procedures scheduled in the protocol
• subject has a known allergic reaction to the medication
• subject is pregnant or breastfeeding
• subject participates in another clinical trial within 30 days prior to study entry or 30 days after end of the study.
• systemic (oral or intravenous) antibiotic treatment against P.a. 14 days prior to the inclusion and during the study