BACKGROUND: Although there have been a number of studies of cheating in universities, surprisingly little has appeared recently in the literature regarding academic dishonesty among medical students. METHOD: To assess the prevalence of cheating in medical schools across the country, class officers at 31 of 40 schools contacted distributed a survey in the spring of 1991 to their second-year classmates. The survey consisted of questions about the students' attitudes toward cheating, their observations of cheating among their classmates, and whether they had themselves cheated. The results were analyzed using contingency tables, t-tests, Pearson correlations, and one-way analysis of variance. RESULTS: Of the 3,975 students attending the 31 schools, 2,459 (62%) responded. Thirty-nine percent of the respondents reported witnessing some type of cheating among classmates during the first two years of medical education, while 66.5% reported having heard about such cheating. When reporting about themselves, 31.4% admitted cheating in junior high school, 40.5% in high school, 16.5% in college, and only 4.7% in medical school. Reports of cheating varied across medical schools, but no relationship was found between rates of cheating and medical school characteristics. Men were more likely to report having cheated than were women. The best predictor of whether someone was likely to cheat in medical school was whether they had cheated before, although the data strongly support the role of environmental factors. Medical school honor codes exercised some effect on cheating behavior, but the effect was not large. CONCLUSION: About 5% of the medical students surveyed reported cheating during the first two years of medical school. The students appeared resigned to the fact that cheating is impossible to eliminate, but they lacked any clear consensus about how to proceed when they became aware of cheating by others. The guidance students appear to need concerns not so much their own ethical behaviors as how and when to intervene to address the ethical conduct of their peers.