A 60-year-old male was admitted to the hospital. He had been well untill two months before admission, when the rash on the anterior chest was found and a diagnosis of lymphoplasmacytoid lymphoma was established by biopsy. One day before admission he experienced high fever and severe low back pain. The white cell count was 700/mm³ with a few atypical cells. Bone marrow picture showed the atypical cells of 96%, a clinical diagnosis of multiple myeloma was made and treatment with prednisolone, 40mg/day, and melphalan, 2mg/day, was begun. Five days later, most rasheds disappeared but the amount of urine diminished and the blood urea nitrogen (BUN) was 99.7mg/dl, creatinine 4.8mg/dl, potassium 5.3 mEq/L, uric acid 13.6mg/dl, phospholus 8.0mg/dl, lactec dehydrogenase ((LDH) 32,100 U/L; a diagnosis of acute tumor lysis syndrome was made. Hemodialysis was carried out, but an association of disseminated intravascular coagulation (DIC), congestive heart failure, and interstitial pneumonia led to a death on the 21st hospital day. From the histochemical characteristics of atypical cells, a final diagnosis was acute monocytic leukemia with tumor formation. Few cases of acute monocytic leukemia has been reported as an underlying cause of acute tumor lysis syndrome, and its induction by small dose of chemothrapy, complicated by DIC and by interstitial pneumonia has been rerely experienced.