In five months of examining some 25,000 blood smears of hospital and clinic patients, 200 patients were found whose blood had hypersegmented neutrophiles without macrocytosis of the erythrocytes. Sixty-five of these patients subsequently received adequate laboratory work-up. Seven proved to have a bacterial inhibitor in their serum which interfered with the microbiological assay of folic acid. Of these patients, six had normal B-12 levels. Of the remaining 58, 45 had a deficiency of folate, of vitamin B-12 or of both. In 13, no such deficiency could be established. Of these, seven proved to be uremic. It is concluded that the additional effort required to search carefully for hypersegmentation, even in the absence of erythroid macrocytosis, is thoroughly justified, and that a large percentage of those patients in whose blood hypersegmented neutrophiles are found will prove to have important deficiencies of folate or B-12, or will prove to be uremic.