Background : The promotion and advertisement of pharmaceuticals should be based on evidence from clinical trials. We conducted this study to assess whether the pharmaceutical advertisement claims in Korean medical journals had relevant references, and whether the claims were supported by the references. Methods : We reviewed pharmaceutical advertisements in five Korean medical journals issued during the first half of 1999 and during the first half of 2004. Three investigators independently reviewed the advertisements to see whether the studies quoted to endorse the advertising messages supported the corresponding claims. Using multiple logistic regression analyses, we investigated which factors were associated with the quality of the advertisement claims. Results : From the 550 advertisements in the five journals, we identified 157 different advertisements and 475 different promotional claims. Only 149 claims had at least one reference, and 105 claims had references of published article. We could find supporting evidences in the 90 claims. The factors which were associated with the quality of advertisement claims were category of drugs, category of claims, and the manufacturer characteristics. Claims for cardiovascular and endocrine drugs, and claims on efficacy, and claims of multinational company were more evidence-based. Conclusion : Majority of the pharmaceutical advertisement claims in Korea did not have appropriate references. Drug category, claim category, and the manufacturer characteristics were associated with the quality of advertisement claims, and the manufacturer characteristics was the most important determinants.