We report a rare case of idiopathic myelofibrosis transformed to acute myelomonocytic leukemia associated with non-Hodgkin's lymphoma. A 64-year-old woman was admitted to our department because of anemia and leukocytosis. On admission, anemia and hepatosplenomegaly were noted. The hemoglobin content was 6.8 g/dl, and WBC count was 26,200/microliters with an increased number of immature neutrophils. Bone marrow biopsy revealed an increased amount of reticulin fiber. Because she had no disease which causes secondary myelofibrosis, idiopathic myelofibrosis was diagnosed, and she was treated with prednisolone, anabolic steroid and blood transfusion. Fifteen months after the diagnosis of myelofibrosis, blast increased in her peripheral blood and her spleen and liver enlarged remarkably. A tumor of right parotid region was recognized at the same time. The pathological diagnosis of biopsied tumor was non-Hodgkin's lymphoma. The cytochemical study of blasts in her peripheral blood showed that she had acute myelomonocytic leukemia. In spite of intensive chemotherapy, she died from heart failure, respiratory failure and renal insufficiency.