At Manchester University medical students (n = 658) had more knowledge than law students (n = 245) about the hazards of smoking, were more convinced by the relevant evidence, and assessed the risk to health more highly. However, there was no significant difference in their smoking behaviour. Among medical students 61% were non-smokers, 10% ex-smokers, and 29% smokers, of whom three-fifths smoked regularly. During their training they were more likely to start smoking than to give it up, and more likely to increase their consumption than decrease it. Expansion of smoking occurred especially in the early years of the course, whereas the main increase in knowledge was in the clinical period. Students' knowledge and their opinion of the evidence were related to their stage of education, whereas their assessment of the risk was linked with their smoking behaviour. Expansion of smoking at medical school may be forestalled by providing information about the hazards as early as possible, although some smokers will continue the habit irrespective of knowledge.