Medical research involving human subjects can be risky and burdensome. Therefore, such research must be reviewed and approved by a Research Ethics Committee (REC). To guarantee the safety of the subjects, it is very important that these studies be conducted in accordance with the approved protocol. An important issue in this respect is whether studies include the requisite number of subjects based on the research question. The research question is unlikely to be answered reliably if the requisite number of subjects (...) is not met. In such cases, subjects are exposed to unnecessary risks and burdens. In this descriptive study, the authors evaluated how frequently studies are completed with the required number of subjects. Moreover, the authors identified the characteristics of research that does and does not include the required number of subjects. The results of this study show that a considerable proportion of studies (41/107) were terminated although they failed to recruit a sufficient number of subjects. Furthermore, the authors found that investigator-initiated studies have significantly (p=0.028) more problems in recruiting the requisite number of subjects than studies initiated by pharmaceutical companies. Potential solutions are discussed to reduce the number of studies that do not include a sufficient number of subjects. (shrink)