OBJECTIVE: The aim of this study was to assess the tuberculin skin test conversion incidence in Swiss medical students. Methods: This was a prospective cohort study of medical students at the University of Zurich, using a standardized tuberculin skin test before and after clinical electives. RESULTS: Two hundred and sixty-two students accepted the invitation to the pre-clerkship test, and 155 (59.2%) subjects were retested after an average period of 2.25 years. An increase of more than 10 mm in the transverse diameter of the induration in the retest compared to the baseline test was observed in 12 (7.7%) students. The annual conversion rate was 3.4% (95% CI 1.8-6.0%). CONCLUSIONS: Even in an industrialized country, the risk of tuberculosis infection--as estimated by the tuberculin skin test--is substantial for health care professionals. Possible explanations for the high incidence and possible bias are discussed.