We compared the age distribution of all adults in New Mexico with cancer incident from 1959 through 1982 with that of all adult New Mexican patients enrolled in cancer treatment protocols sponsored by the Southwest Oncology Group (New Mexico). For all cancer sites, elderly patients were substantially underrepresented in the Southwest Oncology Group protocols. While 31% of all adult patients with cancer were over age 70 years, only 7% of patients with cancer enrolled in Southwest Oncology Group protocols were in that age group. The underrepresentation of elderly individuals in cancer treatment protocols will make it difficult to determine optimal therapies for older patients with cancer.