In cystic fibrosis, a majority of patients develop persistent lifelong respiratory tract colonisation with pseudomonas aeruginosa while a minority do not appear to become colonised. There is some evidence that non-secretion of ABO Blood Group antigens into body secretions is a marker for susceptibility to certain gram negative infections. Therefore, we studied 47 adult patients with cystic fibrosis to identify an association, if any, between secretor status and the presence or absence of pseudomonas aeruginosa in their sputum. Overall, we found no difference between the prevalence of non-secretors in our patient group and in the normal population. Furthermore, there was no association between secretor status and presence or absence of pseudomonas aeruginosa in sputum. We conclude that secretor status is unlikely to play a major role in susceptibility to pseudomonas aeruginosa infection in patients with cystic fibrosis.