Hospitalized patients were studied prospectively in an attempt to determine whether idiopathic eosinopenia in the absence of other major changes in blood cells is rare or is more frequent than is commonly recognized. Strict criteria for eosinopenia were used; patients with more than 10.0 X 10(9)/liter total leukocyte count, or less than 4.0, as well as those with any form of hematologic cancer, or those receiving any form of cancer chemotherapy, were excluded from the study. With those criteria and exclusions, only 24 patients with eosinopenia were found among 24,300. Twenty were receiving some form of adrenal glucocorticosteroid (steroid) and of the other four, three had serious organic diseases for which they were receiving various drugs. The remaining patient, whose primary problem was depression, could have had drug-induced eosinopenia. Thus, unexpected eosinopenia appears to be a very rare event or syndrome. A normal range for eosinophils was defined from studies of 740 medical students, which was 0.015 to 0.65 X 10(9)/liter and was quite similar to previously reported values. The effect of acute or chronic steroid administration on eosinophils in normal human subjects was studied. Confirming studies reported for man and many other mammals, eosinopenia developed promptly, but disappeared within hours unless repeated doses were given. Literature on various types of eosinopenia was reviewed.