Upon reading the recently published article entitled "Adverse pregnancy outcomes with assisted reproductive technology in non-obese women with polycystic ovary syndrome: a case control study" [1], we were compelled to point out that the study design does not conform to the principles of a case-control study. The authors state that the aim of the study was to compare the prevalence of adverse pregnancy outcomes between non-obese women with polycystic ovary syndrome compared to a control group. However, the study participants were selected on the basis of the specific exposure groups; for case control studies participants are selected on the basis of an outcome of interest. Thus, the authors have conducted a cohort study.