A 58-year-old man exhibited polyarthritis, fever, thrombocytopenia and progressive anemia. Undifferentiated connective tissue diseases(UCTD) was diagnosed based on laboratory and radiographic findings. After diagnosis, the patient received glucocorticoid and blood-transfusion. The symptoms were improved notablely. However,the level of hemoglobin was lower than normal(between 57 g/L and 75 g/L). Thrombocytes were 19 000/microl to 32 000/microl. Bone marrow aspiration revealed highly abnormal cell morphology, indicating myelodysplastic syndrome(MDS). A diagnosis of UCTD with MDS was made. The patient was successfully treated by decitabine and thalidomide(an immunosuppressive regimen). It is necessary to promptly examine bone marrow cell morphology and chromosomal aberration in cases with connective tissue diseases complicated by sudden cytopenia and thrombocytopenia.