The clinically detailed risk information system for cost (CD-RISC) contains definitions for several hundred severity-adjusted conditions that can be used to predict future health care costs. We develop a prospective Medicare CD-RISC model using a 5-percent sample of Medicare beneficiaries and data that contain 1996 diagnostic information and 1997 annualized costs. The CD-RISC model has a hierarchical structure that implies that only the most expensive condition-severity variable within a body system affects payments. This minimizes incentives to game the system by entering multiple related codes for the same condition. The R2 for the CD-RISC model is 11 percent.