Blood loss was measured and serial hematocrit readings and reticulocyte counts were performed on 93 consecutive patients. The mean daily blood loss of 54 ml was accompanied by an observed reticulocyte count of 3.4% over an average of 8.2 days. Hematocrit values in 75 patients with no evidence of bleeding decreased 5.7% during the 20.9 days of hospitalization. Of the 64 patients not anemic at the time of admission, 26 (40%) became anemic with no obvious cause prior to discharge from the hospital. Bloodletting for diagnostic studies is a significant cause of anemia and reticulocytosis in hospitalized patients.