In order to identify the possible reservoirs and routes of cross-infection with Pseudomonas aeruginosa, samples were collected during a six-week period in autumn 1992 from patients, their visiting parents, staff and the inanimate environment of the Danish Cystic Fibrosis (CF) Centre and from a control ward with common paediatric diseases. All the P. aeruginosa strains were phage typed and serotyped. From 240 CF patients, 310 strains of P. aeruginosa were isolated, and of these 283 (91.3%) belonged to the polyagglutinable phenotype, most often with a short phage type (31/188 or 109). P. aeruginosa was isolated from only six (0.6%) of 1000 swabs taken from the environment. These six environmental strains and 20 P. aeruginosa strains from CF patients with identical serotype and phage type were examined with pulsed field gel electrophoresis. None of the patients harboured strains similar to the environmental strains, indicating the present isolation procedure and hygienic precautions were effective in our CF centre, and prevented contamination of the environment with P. aeruginosa.