The choice of an appropriate control group has been recognized as one of the most difficult problems in the methodology of case/control studies, both in theory and in practice. In the study of chronic pain, a "well control group" has frequently been employed. Although this design has intuitive appeal, it contains the potential for bias and error. In order to obtain the proper control group, the same exclusion criteria should be applied to cases and controls. Second, controls should be selected independently of exposure to the putative risk factors under investigation. To illustrate the nature of this problem we present two numerical examples, using a chronic pain disorder.