Two hundred and forty Danish patients with cystic fibrosis (97% of the total CF population in Denmark) participated in a point-prevalence study of Pseudomonas aeruginosa infection. One hundred and ninety-two patients were treated at the Danish CF centre and 48 patients were treated in other places. The age distribution was significantly different as no patients older than 19 years were found in the non-centre group. Pathogenic bacteria were isolated from the sputum of 96% of the patients. P. aeruginosa was more prevalent in patients from the centre, whereas Staphylococcus aureus was more prevalent in the non-centre group. No difference in serogroup and phage pattern of P. aeruginosa was found. There was a tendency that non-centre treated patients acquired chronic broncho-pulmonary P. aeruginosa infection later, but at the age of 16 years 90% of all patients will be chronically infected. Chronic P. aeruginosa infection was significantly more common in the age group 10-14 years at the centre than outside the centre. It is not possible to prevent chronic P. aeruginosa infection in CF patients treated in small groups and because of the better prognosis of centralized treatment the latter must be recommended.