A total of 243 medical students in the third term, phase I, the University of Copenhagen, completed a questionnaire about health, health behaviour, including smoking during autumn 1988 and spring 1989. This investigation aimed at elucidation of the smoking habits of the medical students and their attitudes to smoking in the medical profession. Attempts were made to test the following hypotheses: 1. That smoking among medical students occurs particularly frequently among stressed students. The investigation shows that 33% of the smokers frequently felt stressed as compared with 13% of the non-smokers. 2. That medical students smoke less than the remaining population in the same age group. It was found that 22% of the medical students smoke. In the remainder of the population approximately 40% of the persons aged 20-24 years smoked. 3. That smokers consider that smoking is the doctor's own business while non-smokers consider that doctors have a professional responsibility. The investigation shows that 36% of the smokers consider that smoking is the doctor's own business in contrast to 64% of the non-smokers. The main result of the analysis is that smokers and non-smokers resemble one another socially and as regards health behaviour in the majority of respects but that smokers, as a whole, have poorer health behaviour and poorer self-assessed health than non-smokers.