A 61-year-old female was diagnosed with multiple myeloma (MM) in 2001. After treatment with chemotherapy containing alkylating agents and thalidomide, she underwent autologous stem cell transplantation in 2003, with high-dose melphalan as a conditioning regimen. Thalidomide was also given after the transplant from July 2003 to November 2005 for residual disease and she remained in partial remission. In October 2008, she developed pancytopenia. Bone marrow examination confirmed the diagnosis of acute B lymphoblastic leukemia (B-ALL). We performed IgH gene rearrangement studies on genomic DNA which revealed the MM, and ALL seemed to be derived from different clones. The development of MM and ALL in the same patient is a very rare event. Further accumulation of the cases to understand the mechanism underlying this event is warranted.