Evidence-based reasons for segregation of patients colonized with Pseudomonas aerugionsa in the outpatient setting are unclear. To clarify local decisions, Pseudomonas genotyping of the local environment, patients and patient contacts was undertaken in 1993. The hospital environment was re-swabbed in 1997. Pseudomonas genotyping of old and new patients attending the North Staffordshire cystic fibrosis clinic has subsequently been undertaken and more recently been repeated on an annual basis to assess whether the same Pseudomonas genotypes can be found in both the environment and in patients, and whether the same Pseudomonas genotype can be transferred from one patient to another. No Pseudomonas genotype found in the local environment in 1993 or in 1997 has been found in any of our patients. Nine children attending the same special school for many years and sharing the same physiotherapy facilities showed no evidence of cross-infectivity. Except for siblings living in the same household our cross-infectivity rate is very low and where cross-infection has potentially occurred the level of contact between these patients has been minimal. This study does not support the suggestion that patients with cystic fibrosis attending the North Staffordshire clinic and colonized with Pseudomonas aeruginosa should be segregated from non-colonized patients.