Among 249 cases of leukemia examined, there were 88 cases (35. 3%) which were associated with tumor formation. These tumors occurred singly or multiple, in various kinds of leukemias and favorably in the kidney, mediastinum, skin and periosteum. Concerning the frequency of tumor formation, chloroleukemia had the highest rate, followed by monocytic leukemia, while myeloid leukemia showed a considerably lower rate. There was no difference in the frequency of tumor formation between acute and chronic leukemia, nor in relation to the number of mitotic figures of the leukemic cells. Consequently the tumor forming leukemia is hardly regarded as the more malignant form of leukemia, but is to be considered nothing but one type of neoplastic growth of blood cells. Finally it is considered that the condition of the host seems to be significant as the factor, determining tumor formation as well as the sort and character of leukemic cells.