OBJECTIVES: Typing of Pseudomonas aeruginosa isolates from Norwegian cystic fibrosis (CF) patients with chronic Pseudomonas lung infection in order to see whether cross-infection might have occurred. METHODS: Isolates from 60 patients were collected during the years 1994-98, and typed by pulsed field gel electrophoresis. RESULTS: Seventy-one strains were identified. One large cluster of identical strains included 27 patients, and 13 smaller clusters of 2-4 patients were found (26 patients). Seven patients had a strain not shared by other patients (private strains). Harboring the main cluster strain was significantly associated with participation in summer camps and training courses (P = 0.004, chi-squared test). There were no associations with regular admissions to hospital (intravenous antibiotic courses) or smaller social gatherings of short duration. Small clusters and private strains were not associated with any of the risk factors. All strains were sensitive to colistin. The minimal inhibitory concentrations were generally lower in Norwegian P. aeruginosa strains compared with isolates from Danish patients. CONCLUSIONS: Our results indicate that cross-infection with P. aeruginosa between cystic fibrosis patients has occurred.