Despite a plethora of studies on overall attitudes related to organ donation in a health care setting, little is known as to whether or not medical students at various levels of undergraduate training have distinctive attitudes towards donation. The purpose of this study was to analyze attitudes of first- and sixth-year students towards organ donation. A total of 988 students in first (573) and the final, sixth, year (415) were recruited at the Faculty of Medicine, University of Belgrade (Serbia), in the period 2-9 December, 2013. Data were collected using an anonymous questionnaire. There were 3.0% of first-year and 3.9% of sixth-year students who were registered donors (p = 0.019). Sixth-year students felt statistically significantly more positive towards signing an organ donor card [odds ratio (OR) 1.45, 95% confidence interval (CI) 1.13-1.88]. Sixth-year students also considered that organ donation was sufficiently promoted, as well as that organ donation should be practiced unless there is a written notice of objection. In addition, sixth-year students had more confidence in local organ transplant institutions. In a multivariate logistic regression analysis significant predictor of positive attitude for signing organ donor card was being blood donor (OR 1.55, 95% CI 1.03-2.35). Undergraduate training and increase in overall medical knowledge seem to contribute in shaping positive attitudes towards being an organ donor. It would be beneficial that strategies for organ donation promotion target local centers for blood donor recruitment. Nevertheless, further promotion of organ donation is necessary to expand the total donor pool.