During the six months from January-June 1994, 10 cases of severe and 11 of less severe pulmonary infection caused by Pseudomonas aeruginosa were diagnosed in patients with chronic obstructive airways disease. Possible sources were evaluated. P. aeruginosa was isolated from four of the 22 nebulizers tested. The relationship of isolates from the patients and nebulizers was confirmed by sero- and phage-typing, and by arbitrarily-primed polymerase chain reaction (AP-PCR). Three types were identified and the distribution of types in patients with severe infection was as follows (one patient had a multiple infection). Type I was isolated from two nebulizers and from sputa, and/or blood and/or bronchial protected specimen brush samples or bronchial lavage fluid from four patients. Type II came from the sputa of three patients and a third nebulizer; and type III from sputa and/or blood of four further patients and another nebulizer. The data provided evidence for the relation between P. aeruginosa as a cause of infection and the contamination of the nebulizers. When nebulizer mouthpieces were changed every 24 h and sterilized between patients, no more contamination occurred, and the outbreak ceased.