A 67-year-old man was diagnosed with multiple myeloma IgA-lambda type, Durie-Salmon classification stage IIIA in October 2001. He received five courses of induction chemotherapy consisting of vincristine, doxorubicin and dexamethasone and then underwent high dose chemotherapy followed by autologous stem cell transplantation in March 2003. He achieved partial response, but then relapsed after treatment with thalidomide and was admitted to our hospital in June 2007. The patient was complicated by tumor lysis syndrome (TLS) after receiving bortezomib therapy twice. Computed tomography after bortezomib therapy showed the rapid appearance of tumors in the right upper lobe of the lung, tail of the pancreas and the spleen. Though he was treated with antifungal agents, micafungin and voriconazole, he died eighty-five days after admission. Autopsy specimen showed fungal clumps and hemorrhagic infarction in the lung and spleen, and vegetation at the mitral valve was the same fungus as found in the lung. We diagnosed disseminated zygomycosis based on the pathological fungal morphology. This case suggested that metabolic acidosis was caused by TLS, while poorly controlled diabetes, secondary hemochromatosis due to transfusion, and breakthrough zygomycosis during antifungal therapy were thought to be factors contributing to the development of zygomycosis.