A 27-year-old male with systemic lymphadenopathy was diagnosed as lymphoblastic-type lymphoma by inguinal lymph node biopsy in September, 1990. Bone marrow at the initial diagnosis contained 55.4% lymphoblasts with a phenotype of peroxidase (-), CD7 (+), CD4 (-), CD8 (-). Lymphadenopathy and lymphoblasts in bone marrow disappeared after MACOP-B therapy. In December, 1990, however, the patient again noticed swelling of cervical lymph nodes. At this time, the bone marrow contained 36.4% myeloblasts with a peroxidase (+), CD7 (+), CD13 (+), CD33 (+) phenotype. Cytogenetic and genetic study revealed that the lymphoblasts at the initial diagnosis and the myeloblasts at relapse shared an common abnormal karyotype, 11p-, and the same rearranged band of T-cell receptor delta, gamma, beta genes, suggesting that these two blasts originated from the same clone. The blasts obtained from the cervical lymph node at relapse were still negative for peroxidase, in contrast to the blasts from bone marrow. These findings suggest that this leukemia originated from a stem cell and differentiated along multilineage pathways.