Background/Aims: In cystic fibrosis (CF), chronic microbial lung infections are difficult to treat and cause morbidity and increased mortality. Methods: In a multicentre, open-label, exploratory, non-interventional study, inhaled tobramycin (300 mg twice daily) and colistin (1 million I.U. twice daily) were sequentially combined with the aim to investigate the effect on 41 CF patients with chronic P. aeruginosa infections for six months (mean age 24 ± 10.8y). Results: Six patients had adverse events that were assessed as being related to treatment. Mucus production and coughing both decreased in 39%, whereas FEV1 absolute and relative to baseline increased by 4.9% and 9.1%, respectively (p = 0.004) in 29 patients, who were definitely treated sequentially. Efficacy of the therapy was rated ‘excellent' or ‘good' by the physicians in 80.5% of the patients. Conclusions: The results indicate that treatment with inhaled antibiotics, sequentially combined, was very well tolerated by most patients and may have a beneficial effect, even if transitory on lung function and respiratory symptoms.