This cross-sectional study examined medical and dental students' knowledge of preventive medicine. The purpose of the study was to assess the knowledge of preventive medicine--both epidemiological and clinical--that students demonstrate as they enter medical and dental school and acquire during training. The results of a two-group (medical versus dental school) by three-time (first-, second-, and third-year level) analysis of variance of students' mean examination scores showed significant main effects for school (F = 28.3, P less than .001) and training level (F = 24.5, P less than .001), and a significant school-by-training level interaction (F = 13.9, P less than .001). Medical students at higher training levels demonstrated greater knowledge of preventive medicine. In contrast, there were no significant differences by year of training among the dental students on the total test or on the epidemiological or clinical subscales. Both the dental and medical students demonstrated greater knowledge of clinical applications than of epidemiological foundations (t = 4.21, P less than .01). Dental students performed better than medical students on items that focused on diseases and risk factors that manifest symptoms more likely to be observed in dentistry than in general medical practice. The findings underscore the need to demonstrate practice relevance when teaching preventive medicine.