Since the creation of the Organizacion Nacional de Trasplantes (ONT) in 1989, the organ donation rate in Spain has doubled. Although often attributed to improved donor recruitment efforts, this increase may also represent higher utilization of marginal donors. Therefore, age-related donor recruitment in Spain and the US was evaluated. Data from the ONT, the US Scientific Registry of Transplant Recipients (SRTR), the US Census Bureau, and the Tempus databank of Spain's Instituto Nacional de Estadistica (INE) were analyzed. Between 1989 and 1999, the number of donors in Spain increased from 14.3 to 33.7 per million population (pmp; 136% increase) compared with an increase in the US from 16.2 to 21.5 donors pmp (33%). The largest difference between Spain and the US in the increased number of donors was in the 45-year-old group, representing 30.3% of donors in Spain in 1999 (44 donors pmp). If the US increased its older donor rates to match Spain's, an incremental 1235 donors per year would be realized. The high Spanish organ donation rates are largely attributable to increased use of older donors. Utilizing similar proportions of older donors in the US would increase the donor pool by almost 40%.