BACKGROUND AND METHODS: To assess the relation between seasonal variation and the onset of initial and chronic Pseudomonas aeruginosa infection, 300 Danish patients with cystic fibrosis were investigated. A retrospective analysis based on case reports was performed to identify the date and year of initial and chronic P aeruginosa infection from 1965 to 1990. RESULTS: Sixty six per cent of the patients contracted their initial P aeruginosa colonisation and 68% contracted chronic infection during the winter months (October to March). Despite major changes in treatment, including improved and intensified antibiotic treatment, during the investigation period in our cystic fibrosis centre, the seasonal difference in P aeruginosa infection persisted. CONCLUSIONS: As respiratory virus infections have the same seasonal distribution in Denmark such infections may pave the way for P aeruginosa and thus explain the parallel seasonal occurrence of this pathogen in patients with cystic fibrosis.