Bias and validity in surgical clinical trial LOU Wen-hui.Department of General Surgery, Zhongshan Hospital, Fudan University,Shanghai200032,China Abstract There are internal validity and external validity in clinical trial. Internal validity closely related to clinical trial design and execution, which reflects the accuracy of its conclusions about an intervention’s effects on a given group of subjects under a study’s specific circumstances. External validity deals with the applicability of a study’s conclusions to the real world. Bias generated during the design and execution of clinical trial will jeopardize the internal and external validity。These biases may sometimes be ameliorated by randomization, blinding, and intervention standardization. It is helpful for surgeons to understand the reason behind bias, so as to judge the validity of a surgical clinical trial and properly apply the conclusion in real clinical setting.