Initial Pseudomonas aeruginosa infection in patients with cystic fibrosis is a key step in pulmonary evolution of the disease. This first infection is most of the time early in life and gives some practical problems to the clinicians. It is important to look for colonisation with Pseudomonas aeruginosa as often as possible, at least every 3 month, by sputum culture when the child is able to or with induced sputum by hypertonic saline nebulisation. Treatment with inhalation of an antibiotic, preferably tobramycin for a 28 days course. Efficacy of the treatment as to be assessed by new culture.