We report a patient with lung cancer who developed CD56-positive acute lymphoblastic leukemia. He was referred to our hospital for thrombocytopenia. Atypical cells were found in the blood and the bone marrow. These cells were immunophenotypically positive for CD3ε, CD56, and terminal deoxynucleotidyl transferase, and negative for surface CD3, CD4, CD19, CD33, and myeloperoxidase. A small proportion of leukemic cells express CD13. There were no rearrangements of T-cell receptor (TCR)-β, TCR-γ, or immunoglobulin heavy chain. No Epstein-Barr virus was detected. Systemic examination did not detect any tumors other than pulmonary adenocarcinoma, and the patient was diagnosed as having acute natural killer (NK) cell leukemia. Chemotherapy was effective, and he achieved complete remission. The course of the disease was complicated by a lung abscess, and the patient died 3 months after the diagnosis. We considered that the diagnosis was blastic NK cell lymphoma/leukemia subtype. However, it actually was myeloid/NK cell precursor leukemia subtype that weakly expressed CD13.