A study of 196 healthy geriatric females and 26 males confirmed the high prevalence of anemia in this population. Forty-two females (21.4%) had a hematocrit of less than 36% and nine (34%) of the males had a value of less than 40%. A careful evaluation, including a therapeutic trial of oral iron in some subjects, made a diagnosis of iron deficiency anemia in only three and the anemia of chronic disease could be diagnosed in two. No subject had folate or B12 deficiency and hemolytic anemia was not present. Thus, in most of these subjects, the lower hematocrit was not due to commonly recognized causes. Further evaluation revealed a high prevalence of leukopenia in this population; 30% having counts less than 5 X 10(3)/microliter and 10% having values less than 4 X 10(-3)/microliter. Of most importance, leukocyte counts were significantly lower in anemic subjects in whom a high incidence of neutropenia was also demonstrated. This suggests that the mechanisms of the unexplained anemia and neutropenia is an overall reduction in hematopoietic cell numbers. This conclusion is consistent with the observation that many aged subjects have a decreased ability to mount an adequate leukocyte response to infection. These findings suggest that the presence of anemia marks an overall reduction in hematopoietic reserve and provides a clue to those aged subjects of greatest risk of an inadequate response to stress.