Preleukemia is defined as a hematological disorder preceding the clinical recognition of leukemia. Up to now, some cases of preleukemia have reported. However, systematic studies on preleukemia have been rare. This paper describes clinical data of 62 cases of preleukemia in Japan, which may be summarized as follows. 1. Hypoplastic anemia-like hematological disorder (hypoplastic preleukemic stage) was the most frequent type among various preleukemic disorders (41 of 62 cases). Secondly, dyserythropoietic conditions, including pernicious anemia and sideroblastic anemia, were seen in preleukemic disorders (8 of 62 cases). 2. Preleukemia tended to occur in rather elderly males. This tendency was also recognized in hypoplastic preleukemic cases. 3. Preleukemic stages tended to persist for a relatively long period and survival times from the point of diagnosis of leukemia were relatively short. 4. Fifty-one of 62 cases terminated into acute lekemia, of which 31 cases were acute myelocytic leukemia, 7 cases monocytic leukemia, 5 cases acute lymphocytic leukemia, 2 cases acute erythroleukemia, 6 cases unknown type. Chronic leukemia was seen only in 6 cases of 62 cases and they were all myelocytic. Particularly, all hypoplastic preleukemic cases, excluding 4 unknown cases, terminated into acute leukemia. 5. The appearance of erythroblasts in the peripheral blood, a slight increase of myeloblasts and a relative erythroid hyperplasia in the bone marrow were noted at the hypoplastic preleukemic stage. On the other hand, some of hypolastic preleukemic cases tended to show aleukemic finding and hypocellular marrow at the time of diagnosis of leukemia.