Smoking behaviour and prevalence rates among medical students and medical professionals are important public health issues, as physicians' attitudes and interventions are decisive for the patients' success in quitting smoking. Studies dealing with prevalence rates of smoking usually use only face-to-face interviews or self-administered questionnaires, which may induce vague findings. Additional measurement of exhaled carbon monoxide is an objective, easy, immediate, non-invasive and inexpensive mode of indicating smoking behaviour and will complement and at some stage replace the usual question regarding the number of cigarettes consumed. CO-measurement of 260 medical students was taken during compulsory public health training at the Medical University Vienna. Definite indication of active smoking was found in 12% of the students, 9.5% showed CO-levels between 6 and 10 ppm and 78% were definitely non-smokers with a CO level between 0 and 5 ppm. The students had the opportunity to get to know an important diagnostic technique and additionally learned about their own smoking habits.