Organ transplantation has become a viable treatment for an increasing number of patients suffering from irreversible organ failure. In response to the steeply rising demand for transplantation, both the number of transplant centers and the number of patients on waiting lists have grown rapidly. Because organ donation has not kept pace with demand, each year a greater number of patients die while awaiting donor organs. (About 9% of all patients on the list in 1993 but not transplanted died. Death rates were highest, 19% and 16% respectively, for patients awaiting hearts and livers.) Among the factors contributing to the organ shortage are cultural and psychological barriers to donation and missed opportunities to request donation. An accompanying diminution in traumatic deaths of potential young donors has made older and other marginal, or higher-risk, donors the focus of studies on expansion of the donor pool. The studies reviewed herein evaluated donor risk factors such as age, disease (including infection), obesity, cold ischemia time, suboptimal organ function, and nontraumatic causes of death. Overall, broadened criteria for acceptable donor kidneys, hearts, and livers appear to lessen graft survival rates somewhat compared with rates for ideal donor organs. Nonetheless, use of higher-risk organs allows lifesaving transplants that could not otherwise be performed and results in acceptable prognoses for survival. Further research is needed to identify better tests for evaluating donor organs, provide longer-term follow-up of recipients of higher-risk organs, and develop alternative means to fill the donor-organ shortfall.