The use of sterile plastic sleeves to protect pulmonary artery catheters (PAC) may decrease infection risk. The catheter may require manipulation but contamination of the sleeve risks inoculating organisms into the patients. We sought to determine whether the sleeve remains sterile and for how long. We conducted a prospective observational study to culture the components of the PAC in a critically ill population. Upon removal we cultured 1) the PAC tip, 2) the PAC introducer exit site, 3) the PAC introducer hub and 4) a sterile irrigant solution which ran down the inside of the protective sleeve. Demographic, catheter and disease variables were also collected. There were 102 PAC episodes amounting to 3952 catheter hours with a mean duration of catheterisation of 39.1 (24.2) hours. There were 17 positive culture results, of which six were positive sleeve fluid cultures. In three of these patients the exit site was culture positive, and the same organism was isolated as within the sleeve. No patient had a positive blood culture. There was no difference between those with and without a positive sleeve culture in terms of demographic or disease variables. The protective sleeve of the pulmonary artery catheter does not remain sterile and should not be considered as a sterile barrier. We believe that manipulating the PAC within the sleeve carries the risk of inoculating the patient with pathogenic organisms. The duration of sterility remains to be determined.