The long-term effects of a first-year medical school course in disease prevention focusing on cancer were studied. The authors hypothesized that students' increased knowledge of disease prevention principles would result in more positive attitudes toward aggressive treatment and prevention of cancer and a more frequent use of disease prevention techniques in clinical settings. On two occasions one to three years after the course, groups of students who took the course and groups that did not take the course responded to a test of knowledge of cancer, a cancer attitude survey, a clinical practice survey, and an evaluation of the course's relevance. Also, their clinical behavior was observed and coded during videotaped interviews of patients. While the students who took the course were more knowledgeable about cancer than their peers who did not, they did not have significantly more positive attitudes; nor were they significantly more likely to use prevention techniques in clinical settings. However, the more knowledgeable students--both those who took the course and the control students--had more positive attitudes than the less knowledgeable students. Those students who considered the course relevant to their training were more knowledgeable and had more positive attitudes toward cancer prevention than those who did not consider it relevant.