OBJECTIVE To review published articles in four U.S. obstetrics and gynecology journals labeled "case-control" studies to estimate the frequency of mislabeling the type of study. METHODS I searched PubMed from January 1970 through May 2009, using journal name and "case-control" in the title as search terms. The journals included the American Journal of Obstetrics & Gynecology, Fertility and Sterility, Journal of Reproductive Medicine, and Obstetrics & Gynecology. I reviewed the methods of each report to confirm the study design and calculated the proportion of articles labeled as "case-control" in the title that were not case-control studies. I calculated Fisher's exact 95% confidence intervals around these proportions. RESULTS In the 124 reports identified, the proportion of mislabeled "case-control" studies was 30% overall. It varied from 13% to 36% in the four journals, a 2.8-fold difference in frequency (95% confidence interval 0.9-9.0). The frequency of this mislabeling appears to have increased over time. CONCLUSION Retrospective cohort studies are often mislabeled as "case-control" studies. This misleads readers as to what was done. Researchers need better training in methods and terminology, and editors and reviewers should scrutinize more carefully manuscripts claiming to be "case-control" studies. LEVEL OF EVIDENCE III.