We report an 8-year-old boy with Ph-positive chronic myelogenous leukemia (CML) who showed a testicular tumor as the first manifestation of blastic crisis. Pathological examination of the excised testis revealed marked infiltration of immature cells, whereas bone marrow remained in the chronic phase. Immunohistochemical and molecular examinations disclosed the immature cells to have B-lymphocyte surface determinants and immunoglobulin JH rearrangement. DNA obtained from the testis and peripheral blood cells showed similarly rearranged bcr configurations, indicating that infiltrating cells in the testis originated from the CML clone. Bone marrow showed B-lymphoid blastic crisis 4 months after testicular involvement.