CO-Measurement of medical students was taken during a compulsory public health training at the University of Vienna. The students were not informed previously, so a non-response bias was excluded. Measurements were done with Bedfont EC50-MICRO Carbon monoxide monitor. The cut off point was set at 11 ppM. One hundred and seventy-three students were measured. Define indication of active smoking was found in 9%. With exception of one student all the others with carbon monoxide over 10 ppM called themselves active smokers. CO measurement will replace at some stage the usual question regarding the number of cigarettes consumed. Students also had the opportunity to learn a new diagnostic technique.