The authors describe a case of undifferentiated rhabdomyosarcoma from unknown primary site, presenting as an acute hematologic malignancy with generalized lymphadenopathy, extensive bone marrow involvement and clinical and laboratory features of disseminated intravascular coagulation. Such a peculiar behaviour is known for rhabdomyosarcoma but is rare and can be a serious diagnostic problem for the clinician and the pathologist. The importance of a large spectrum immunohistochemistry as first diagnostic approach to any undifferentiated small-cell malignant tumor is stressed, together with the knowledge of the different immunoreactivity patterns. Desmin, MS-actin and myoglobin are the most reliable markers of this type of myogenic sarcoma.