In chronic myelogenous leukemia (CML), lymph node swelling is generally considered an early sign of blast crisis. We encountered a rare case of CML showing marked lymph node swelling, which represented extramedullary involvement composed of cells at different stages of maturation but not showing blast crisis. In 1996, a 60-year-old Japanese male was diagnosed with CML in the chronic phase, and imatinib mesylate was prescribed following interferon-alpha treatment in 2004. However, the patient chose to discontinue out patient treatment 1 year later. He was referred to our hospital because of severe fatigue in August 2006, and laboratory tests showed a WBC count of 347.9 x 10(9)/l with 6.5% blasts. On bone marrow tests, Ph chromosome presence was found in all cells examined, but there was no additional chromosomal abnormality. Cytogenetic analysis revealed that 82% of cells in on employing FISH showed BCR/ABL. The patient had swelling of the left cervical and both inguinal lymph nodes. A lymph node biopsy showed proliferation in three lineages of hematological cells at different stages of maturation. He was diagnosed with CML in the chronic phase. He was given imatinib mesylate and hydroxyurea, and achieved hematological remission. Sixteen months later, he developed blast crisis and died of cerebral hemorrhage. Such a case of extramedullary involvement composed of cells at different stages of maturation is rare, and suggests the need to biopsy extramedullary tumors in order to evaluate the clinical phase.