The term “atypical leukemia” covers a wide range of heterogeneous disorders whose classification has hitherto been controversial. This controversy has arisen from the use of different criteria for the definition of the various entities. The application of morphological criteria has led to the separation of conditions without recognizable leukemic blasts, e.g., refractory anemia [11] and refractory anemia with ring sideroblasts [11], from subentities with a definite leukemic population, for which the terms refractory anemia with excess of blasts (RAEB) [38], oligoblastic leukemia [81], and hypoplastic acute leukemia [10, 78, 132] are used. Laying the emphasis on the disease stage at clinical presentation has allowed the differentiation of preleukemic states in which the diagnosis of leukemia has not yet been possible because of the lack of a definite leukemic cell population from overt acute leukemia [16,17, 62,114, 206]. Observation of the disease course has allowed separation of a further group of patients with a slowly progressing course, in whom the terms “subacute leukemia” [30] and “smoldering leukemia” [44, 74, 75, 83, 194] have been applied. The combination of morphological and anamnestic criteria, finally, has led to the definition of “subacute” and “chronic myelomonocytic leukemia” [11,210].