Objectives: To analyze the concept of “case series” in the medical literature compared with case reports. Methods: A PubMed search for articles published during 2009 which had “case series” in their title was performed. A total number of 621 articles were retrieved. 586 papers were included in the analysis and 35 were excluded (18 were commentary letters, 5 were not in English, and twelve could not be retrieved by our Library). The number of patients and category of these articles were analyzed. Results: The median (range) of the number of cases of articles having “case series” in their title was 7 (1-6432) cases. 186/586 articles had less than 5 cases (31.7%, 95% CI (28.3-35.1%)). The median (range) of the number of cases of articles having “case report” as their publication type was 4 (1-178) cases. Out of the 219 articles categorized as case reports 114 (52.1%, 95% CI (45.6-58.6%)) had less than five cases. Conclusions: The concept of “case series” is not well defined in the literature and does not reflect a specific research design. We suggest that a case series should have more than four patients while four paitents or less should be reported individually as case reports.

Key words: Case report, case series, concept analysis, research design