A 74-year old woman had been suffering from chronic idiopathic myelofibrosis (CIMF) for three years before noticing skin rash and subsequent muscle weakness. On admission, purplish and erythematous skin rash was seen spreading over large parts of her body, including the face, chest, back and extremities. She could not stand up without assistance due to weakness. On laboratory examination, her hemoglobin was found to be 11.8g/dl, platelet 17,000/microl, WBC 22,500/microl (with blast cells), and CK 1,757 IU/I. Auto-antibodies including antinuclear antibody, Jo-I antibody and rheumatoid factors were negative. Abdominal CT revealed giant splenomegaly. She was diagnosed as having dermatomyositis (DM) associated with CIMF. Although administration of prednisolone followed by methyl-prednisolone pulse therapy ameliorated the weakness and skin rash, WBC increased to 35,000/ microl. In case of worsening of CIMF, azathioprine (AZP) administered. This decreased the WBC count to 13,700/microl and the CK to 49 IU/I 40 days after the administration. Patients with CIMF have an increased incidence of complications of other autoimmune diseases, indicating that the immunological mechanism plays some roles in the progression of the disease. There has been only one other reported case of DM complication. This case suggests usefulness of AZP treatment combined with prednisolone in these patients.