The juvenile type of chronic myelogenous leukemia (CML) is characterized by hemorrhagic diathesis, hepatosplenomegaly, generalized enlargement of lymph nodes, reduced erythro- and thrombopoiesis, leukemic infiltration of bone marrow, and decreased activity of leukocyte alkaline phosphatase. The disorder differs from the adult type by the early manifestation of anemia and thrombocytopenia, the minor degree of leucocytosis, the failure to demonstrate the Philadelphia chromosome, and the presence of fetal markers in the patients' erythrocytes. The persistence of fetal markers in erythrocytes and the clinical manifestation in infancy and early childhood suggest that the disease is congenital in origin. The occurrence of CML of the juvenile type in an 8-year-old boy, which is reported in this paper, favors a postnatal acquisition of the disorder. The significance of hematological and cytogenetic findings and new aspects of nosological classification are discussed.