A solitary cutaneous or subcutaneous mass in lymphoblastic lymphoma (LBL) is a rare manifestation. A 15-year-old girl presented with a subcutaneous LBL on her left back. There were no other lesions. Complete remission (CR) was achieved after 2 courses of ACOMP-B (doxorubicin, cyclophosphamide, vincristine, methotrexate, prednisolone, and bleomycin) therapy. Two courses of the chemotherapy and 4 sessions of prophylactic intrathecal methotrexate (15 mg/body) and hydrocortisone (25 mg/body) were added after CR. However, the patient relapsed with bone marrow involvement after 14 months remission. It is necessary to accumulate more experience with this kind of case to find the appropriate treatment strategy for solitary cutaneous or subcutaneous LBL.