In order to identify the possible reservoirs and routes of cross-infection with Pseudomonas aeruginosa, samples from patients, staff, and the environment of a cystic fibrosis centre and two control wards at an infectious disease clinic were collected during a two-week period in 1980. All the Ps. aeruginosa strains were phage and serotyped. Ps. aeruginosa was isolated from 90 (51%) of the cystic fibrosis patients and most belonged to the 0-3/9 complex, characteristic of strains from patients in the centre. Some of the patients were able to spread Ps. aeruginosa into the air and to their hands by coughing, and Ps. aeruginosa in dried sputum could survive for at least one week. Strains of the same epidemiological types as found in the cystic fibrosis patients were isolated from sinks, soap, baths, toys, tables, brushes, cloths, and air in the clinic. In contrast, Ps. aeruginosa of the same epidemiological types were only found in a few of the sinks in one of the control wards where a few cystic fibrosis patients were regularly treated in isolation cubicles. The precautions employed to prevent future cross-infection include segregation of Ps. aeruginosa-infected from non-infected patients in separate wards and arranging for visits on separate days in the out-patients clinic. The survival of cystic fibrosis patients treated in the centre is much longer than those treated outside the centre despite the problems of cross-infection.