Pseudomonas aeruginosa is known to be the most important pathogen in CF, and is associated with increased morbidity and reduced life expectancy. Although we are convinced that peer contacts are psychologically beneficial for patients dealing with CF, we wanted to ensure that our patients did not experience more harm than benefit from these contacts, by patient-to-patient transmission of bacteria. Therefore, we set up 2 studies to evaluate the risk of patient-to patient transmission: a pilot study in the rehabilitation centre of De Haan (2001-2002) and a national study, in collaboration of all 7 Belgian CF centres (2003-2004). From both studies we could derive that the majority of P. aeruginosa colonised patients carried a unique genotype, though small clusters of patients with the same genotype could be described. During the study period in the rehabilitation centre there were 3 patients for whom persistent cross-infection could be suspected. We considered this risk comparable with the risk of getting infected from the environment. Both studies showed that, in the individual patient, morphologically different looking colonies often had the same genotype. The national study revealed that most patients (80%) continue to carry the same strain, when genotyped one year later. The patients carrying a ‘cluster’genotype, had a significantly higher questionnaire score for social contacts than those with a ‘unique’ P. aeruginosa genotype. This led to the conclusion that the ‘cluster’genotypes are acquired from patient contacts in the past, which supports the need for segregation. We also described probable patient-to-patient transmission of A. xylosoxidans in the rehabilitation centre population, with only four genotypes that were identified from 102 isolates of 13 patients. A. xylosoxidans is an emerging pathogen in older CF patients. Because litlle is known about the pathogenicty of the organism, we studied the prevalence of A. xylosoxidans in our CF population and the possible pathogenicity of this organism in a ...