On the basis of the evaluation of laboratory findings in 102 cases including 20 cases which were diagnosed as very early stage chronic myelogenic leukemia (CML), chronological orders of the histopathological pictures of CML were shown as follows; myeloid occupation with Ph/sup 1/-positive cells (leucocyte count 10/sup 10//L)--basophilic leucocytosis thrombocytosis and low activity of neutrocyte-AP (1 - 2 x 10/sup 10//L)--appearance of immature granulocytes more than 5% in the peripheral blood (2 x 10/sup 10//L)--increase of serum Vit. B/sub 12/ (2.5 x 10/sup 10//L)--splenomegaly (5 x 10/sup 10//L)--appearance of subjective symptoms (7 x 10/sup 10//L or more). The relation between an actual number of Ph/sup 1/-positive cells and time of their appearance (months) can be expressed as log Y = 0.053 + 10.023. It was estimated that it takes about 8 years since one Ph/sup 1/-positive cell appears in the body until the leucocyte count becomes 10/sup 11//L and the patient consult a doctor. The idea that Ph/sup 1/-positive cells can be transformed to more malignant clones is helpful to understand the pathological feature of acute transformation of CML.