A pilot study was carried out in order to evaluate the feasibility of determining the seroprevalence rate of HIV infection and the prevalence of risk factors among selected hospital patients at the Centre Hospitalier Universitaire Vaudois (CHUV). Consent was obtained and only blood already obtained for other purposes was used for testing. Among 850 patients between 16 and 65 years of age and residing in Switzerland who were originally selected at random, only 200 patients were accessible for an interview and for whom blood was available in order to be included in the study. Of these, 165 (83%) accepted their blood to be tested. A seropositive result was identified in 2 patients already aware of their serostatus. The age distribution, diagnosis and behavioral risk factors (when available through the interview) were not different among those accepting the test and the ones refusing. However, foreigners were more likely to refuse the blood test than Swiss nationals, even though none of them came from a country with a high rate of endemicity for HIV infection. Reasons for refusal included the following: lack of risk factors 10 (29%), doubts about confidentiality 6 (17%), blood test already performed 5 (14%), language barrier 4 (11%) and others 10 (29%). We conclude that even though the data in our sample did not suggest that patients refusing the test were at an increased risk of being seropositive, it is unlikely that with such a refusal rate, a reliable measure of seroprevalence could be determined in the population of study with the methods used, especially when one is expecting a relatively low seroprevalence rate.