Medical students from 13 medical schools in England, Scotland and Wales were asked about their use of tobacco, alcohol and other drugs. Data from 1,278 students (68% response rate) are presented here. 10% of the respondents were current smokers, 10% ex-smokers. In a typical week 17% of the respondents exceeded the recommended limits for sensible drinking. Other drugs besides tobacco or alcohol had been used by 37% of the respondents. Most of the students who had used drugs had done so on no more than 10 occasions. 9% were current users. Although the majority of medical students neither smoke nor drink heavily nor take drugs, a significant minority either persist in or develop potentially harmful substance use behaviour during their studies. The scope for preventive efforts is considerable.