This article presents a system under consideration by the Health Care Financing Administration (HCFA) for incorporating a measure of severity of illness into the Medicare diagnosis-related groups (DRGs). DRG assignment is one of the main factors in determining the payment made for hospital inpatient services furnished to Medicare beneficiaries. Specifically, the formula used to calculate payment for a single Medicare hospital inpatient case takes an average payment rate for a typical case and multiplies it by the relative weight of the DRG to which it is assigned. Thus, it is easy to see that the DRG relative weights have a large impact on the payment a hospital receives. In this article, we describe the Medicare DRG prospective payment system (PPS), evaluate the various classification elements available for assessing severity of illness, describe the analyses used in formulating this proposal, and present the proposed DRG severity system.