In the past several years, hospital staffs and insurance companies have bombarded all of us with medical necessity questions. Well, there's a new sheriff in town, and its name is severity of illness (SOI). It describes the seriousness of our patients' illnesses and the risk of adverse outcome to treatment choices. We may think of risk stratification as one way of determining which antibiotics to prescribe under certain circumstances. Or it may refer to whether a patient is a candidate for surgery, chemotherapy, or any other type of treatment. The Society of Thoracic Surgeons has produced a methodology for developing an SOI database so that its members can level the playing field and compare data with respect to mortality from chest cases.