Pathogenesis of atypical leukemia (hypoplastic leukemia and smoldering acute leukemia) is still unknown. The author intended to find the factors inducing its unique clinical course by comparing 5 cases of atypical leukemia with the same numder of typical acute leukemia with respect to their blasts cell size, laveling index with (3)H-thymidine and pattern of tissue growth by the bone marrow culture method devised by us. And obtained the following results. 1) The size of 100 blasts chosen randomely out of the bone marrow smear prepared from 5 cases of atypical leukemia was compared with those of typical acute leukemia. The mean val-ue of the former group was 41.9±12.3, while that of the latter was 61.4±16.7. There fore, the size of blasts in the atypical leukemia proved to be somewhat smaller than those of the typical acute leukemia. 2) The mean value of laveling index with (3)H-thymidine of the blasts in the bone marrow of the former group was 6.4% , while that of the latter was 12.9% . 3) The growth pattern of the bone marrow tissue from the 5 patients with atypical leukemia was as follows: The bone marrow explants from these patients were generally hypoplastic, consisting of an increase of fat cells intermingled with several foci scattered made of leukemic blasts. The growth zone revealed much lower cell density than that of the typical acute leukemia. The margin of the growth zone, however, sharply bordered and similar to the pattern of typical acute leukemia. Cells observed predominantly in the growth zone composed of immature leukemic cells and mature lymphocytes throughout the culture period of 24-48 hours. From these results, it is assumed that most of leukemic blasts in the bone marrow of the patients with atpical leukemia are "dormant cells", i.e. belong to a nondividing compartment, and reduced multiplication rate is closely related to their mild clinical courses.