A 3-hour seminar on tobacco was introduced to second year (pre-clinical) medical students in Hong Kong in 1994. The differences in knowledge and attitudes were measured by a self-administered and anonymous questionnaire with 14 items before the seminar (n = 145), and again 2 weeks after the seminar (n = 151). The students also completed an evaluation form at the end of the seminar. Before the seminar, the students were most deficient in their knowledge on the exact magnitude of the risks from smoking and on the risks from smoking relative to the risks from air pollution and asbestos. After the seminar, their knowledge increased significantly (P < 0.005). As for attitudes, in the pre-test 35% strongly agreed that tobacco advertising should be completely banned, and 50% did so in the post-test (P = 0.02). The corresponding figures for banning of all forms of tobacco promotion were 26% and 43% (P < 0.005). In the pre-test, one in four students strongly disagreed that doctor's advice to their patients to stop smoking is totally ineffective, with this proportion increasing to 70% in the post-test (P < 0.005). The majority of the students stated that the seminar was useful. The preclinical medical curriculum should, at the very least, include a tobacco seminar. Our survey shows that it is effective in improving students' knowledge and attitudes on tobacco control.