Duplicate publication is considered unethical. It has several negative impacts. To estimate the frequency and characteristics of duplicate publications in Korean medical journals, we reviewed some portion of Korean journal articles. Among 9,030 articles that are original articles indexed in KoreaMed from January to December 2004, 455 articles (5%) were chosen by random sampling. PubMed, Google scholar, KMbase, and KoreaMed were searched by two librarians. Three authors reviewed titles, abstracts, and full text of index articles and suspected articles independently. Point of disagreement were reconciled by discussion. Criteria for a duplicate publication defined by editors of cardiothoracic journals and International Committee of Medical Journal Editors were used. A total of 455 articles were evaluated, of which 27 (5.93%) index articles were identified with 29 duplicate articles. Among 27 index articles, 1 was quadruple publication and 26 were double publications. Of 29 duplicated articles, 19 were classified as copy, 4 as fragmentation, and 6 as disaggregation. The proportion of duplicate publications in Korean medical journals appears to be higher than expected. Education on publication ethics to researchers is needed.