Very often criteria by which subjects are selected for epidemiological studies are associated in some manner with their health. The Healthy Worker Effect (HWE) or Healthy Person Effect (HPE) is well known. Little has been said about the converse case in which selection is associated with decreased health status, the Sick Person Effect (SPE). The SPE may introduce a bias for some cohort, most clinical follow-up, and some case-control studies when risks are standardized against an inappropriate referent. We demonstrate the existence of the SPE in two studies. Study 1 compares the incidence of a number of different diseases among individuals who were selected as children for medical treatment with that among their siblings. Study 2 computes the Standardized Morbidity Ratios (SmRs) for various acute and chronic diseases for individuals who have reported particular chronic symptoms. The SPE is clearly apparent for all instances where the general population is taken as the referent. The HPE and SPE may present serious problems for the validity of conclusions with respect to risk levels.