Background. Outcomes of expanded criteria donor (ECD) kidney transplants are known to be superior to dialysis but inferior to transplant with a standard donor. Because of recent policy changes, ECD kidneys will be offered only to patients who have agreed to consider such an organ in advance. There is wide variation in opinion concerning the composition of ECD wait lists. However, the relative benefits of accepting an ECD versus waiting for a standard donor have not been quantified. Methods. A Markov model was developed to determine when an individual patient should accept or reject an offer of an ECD kidney to optimize their personal expected quality-adjusted life years (QALY). Input variables were estimated from the United States Renal Data System (USRDS) database using a sample of 35,030 recipients. Results. Recipients of ECD kidneys waited 77 days longer for transplant than recipients of standard donors. The average patient could wait 3.2 years longer, in addition to the time they have already waited, for a standard donor than an ECD and expect equivalent QALYs. Selected subsets revealed differences in wait times that equated QALYs for ECD and standard donors: African American, 4.4 years; age under 30, 4.0 years; age over 60, 11 months. Conclusions. Existing policy is likely to be in the best interests of only certain sets of patients awaiting cadaveric kidney transplantation unless ECDs dramatically reduce expected waiting for transplantation. This is most possible in elderly patients because of the short wait-time reduction required to make ECDs beneficial. Data reported here have been supplied by the USRDS. The interpretation and reporting of these data are the responsibility of the authors and in no way should be seen as an official policy or interpretation of the US Government. The data and analyses reported in the 2001 Annual Report of the United States Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients have been supplied by the United Network for Organ Sharing and University Renal Research and Education Association under contract with Health and Human Services. The authors alone are responsible for reporting and interpreting of these data.