Nasal carriage is an important risk factor for Staphylococcus aureus infection, particularly in HIV-infected individuals. In this analytical cross-sectional study, a variety of probable risk factors associated with nasal carriage of Staphylococcus aureus were investigated. HIV-infected patients were examined within a larger cohort of infectious diseases patients. Staphylococcus aureus strains from HIV-infected and non-HIV-infected carriers were identified by molecular biological analysis. One hundred seventy infectious disease patients, 47 of them infected with HIV, were included. All patients were admitted to the University Hospital of Vienna, Austria, between January and July 1999. Independent significant effects on Staphylococcus aureus nasal carriage were found to be HIV status (OR 2.5, 95% CI 1.1-5.6; P=0.0303), history of operation or severe wound within 3 months prior to admission (OR 4.0, 95% CI 1.3-13.0; P=0.0208), presence of an intravenous device within 2 weeks prior to admission (OR 10.8, 95% CI 2.0-59.4; P=0.0065), and intake of antibiotics within 2 weeks prior to hospitalisation (OR 0.2, 95% CI 0.09-0.6; P=0.0016). Molecular analysis of the Staphylococcus aureus strains revealed that the strains in both groups resembled those of healthy nonhospitalized carriers in the community.