Recently some cases of atypical leukemia have been reported. The term of atypical leukemia is usually applied to cases with aleukemic findings, hypoplastic or hypocellular marrow and a low percentage of leukemic cells. On the other hand, some cases with a benign course were reported as smoldering acute lenkemia, It has been already ascertained that hypoplastic preleukemic stage is different from so-called hypoplastic anemia and the leukemic process is progressing latently at that time. However, whether this stage can be seen in all leukemic cases or in only some limitted types is still unknown. In this paper, this problem was studied through clical and hematological examinations of 7 cases with hypoplastic preleukemic stage, 22 cases of atypical leukemia and 80 cases of typical acute myelocytic leukemia. 1. Hypoplastic preleukemic stage and atypical leukemia tended to be more frequently seen in elderly males. In these 2 groups the absence of splenomegaly was thought to be a characteristic finding compared with typical leukemia. 2. Ten of 22 cases of atypical leukemia were treated under the clinical diagnosis of hypoplastic anemia untill the diagnosis of leukemia was made in our clinic. Retrospective examinations of bone marrow smear were done in 5 of these 10 cases. One of these cases was diagnosed to be leukemia at that time. Four cases denied to be leukemia, however, they were thought to have hypoplastic preleukemic stage at that time because of the presence of a slight increase of myeloblasts or a left shift of granulocyte series and a relative erythroid hyperplasia. On the other hand, these preleukemic changes were not observed in cases of typical leukemia. 4. Most of hypoplastic preleukemic cases and atypical leukemia showed atypical hematological findings during all their clinical courses. 5. Median survival times from the presumptive onset of leukemia and from the diagnosis of leukemia were 13.5 months and 3.3 months in 7 cases with hypoplastic preleukemic stage (excluding one unknown case). In 22 cases of atypical leukemia (excluding 3 unknown cases) they were 10.0 months and 4.5 months, respectively. On the other hand, most patients with typical leukemia treated with the same agents as those used for the above 2 groups died within 3 months. These results suggest that not all leukemia is not preceded by hypoplastic preleukemic stage and also suggest that there may be a close relationship between this stage and atypical leukemia described as "hypoplastic leukemia" and "low percentage leukemia". Furtheremore, these cases included hypoplastic preleukemia and atypical leukemia may correspond clinically to the so- "called smoldering acute leukemia."