Pseudomonas aeruginosa is one of the most important bacterial pathogens in cystic fibrosis-associated lung disease. The means by which the organism is acquired or nosocomially spread is uncertain, but person-to-person spread has been implicated. Therefore, typing of P. aeruginosa from cystic fibrosis patients is necessary to determine the source of infection. Although P. aeruginosa strains can be compared using a variety of nucleic acid typing methods, this is time-consuming, expensive and requires specialised equipment, and is better suited to large reference laboratories. In this study, the use of antimicrobial sensitivity patterns in the investigation of possible cross-infection at cystic fibrosis clinics was examined, and 496 P. aeruginosa isolates from 69 patients were analysed. Analysis of dates and times of hospital visits allowed the study to concentrate on specific contact episodes where cross-infection could have occurred. Genotypic analysis of a number of organisms allowed the value of the sensitivity patterns to be assessed. No evidence of cross-infection was found, but antimicrobial sensitivity patterns proved unreliable in detecting similarity among P. aeruginosa strains.