It has been ascertained that hypoplastic preleukemic stage is the most frequent type among various preleukemic conditions in Japan. However, there are some problems to be solved; namely, (I) What hematological findings are characteristic at this stage? Could hypoplastic preleukemic stage is differentiated from so-called hypoplastic anemia? (II) Whether or not normal hematopoietic cells will become malignant in the course of so-called hypoplastic anemia; in other words, hypoplastic anemia switches over to leukemia? (III) Whether or not a case with hypoplastic preleukemic stage has some potentialities to be leukemia; i,e., this stage is the hematological condition appearing to be hypoplastic anemia although the leukemic process is progressing latently. In this paper, these problems were studies through clinical and hematological examinations of 82 cases of hypoplastic anemia and 6 cases with hypoplastic preleukemic stage. 1. Characteristic hematological findings of so-called hypoplastic anemia are pancytopenia and hypocellular marrow. On the other hand, followings were thought to be atypical hematological findings; (I) reticulocytosis (above 20 ‰ ), (II) an increase of basophil (above 2 % ), (III) an increase of eosinophil (above 6 % ), (IV) monocytosis (above 11 % ), (V) appearance of erythroblasts, immature cells of granulocyte series and undifferentiated cells in the peripheral blood, and (VI) a relative erythroid hyperplasia (above 50 % ), (VII) a slight increase of myeloblasts (above 3 % ), (VIII) a left shift of granulocyte series (promyelocyte, above 12 % ), (IX) appearance of undifferentiated cells in the bone marrow. 2. Hypoplastic anemia were classified into 4 groups; (I) typical hypoplastic anemia without atypical finding, (II) Type I atypical hypoplastic anemia with one atypical finding, (III) Type II atypical hypoplastic anemia with 2 atypical findings, and (IV) Type III atypical hypoplastic anemia with more than 3 atypical findings. The frequency of these 4 types of hypoplastic anemia was 72.1 % , 19.8 % , 5.8 % and 2.3 % , respectively. 3. Five and 10 years' survivors were 25 and 13 of typical hypoplastic anemia, respectively. On the other hand, they were 8 and 3 of Type I atypical hypoplastic anemia. There were no 5 and 10 years' survivors in Type II and III atypical hypoplastic anemia except only one 5 years' survivor in Type III atypical hypoplastic anemia. These data suggested that the prognosis of atypical hypoplastic anemia, particularly Type II and III, were poor compared with typical hypopl
astic anemia. 4. Some cases, which should be differentiated from pernicious anemia and hypoplastic pre
leukemic stage, were present among Type II and III atypical hypoplastic anemia. These data suggest that some other hematological disorders may be included in atypical, particularly Type II and III, hypoplastic anemia. 5. Most of hypoplastic preleukemic cases showed some atypical findings on admission. Among various atypical findings, a slight increase of myeloblasts or a left shift of granulocytic series complicated with a relative erythroid hyperplasia should be thought to be characteristic at hypoplastic preleukemic stage. These hematological findings were rarely seen in so-called hypoplastic anemia. From these results, it is suggested that hypoplastic preleukemic stage may differ from so-called hypoplastic anemia and it is also suggested that the leukemic process may be progressing latently at that time.