We report an extremely rare case of interdigitating dendritic cell sarcoma/tumor (IDCS). A 52-year-old man presented with a 2-week history of fever in January 2002. Physical examination revealed enlarged, painless right axillary lymph nodes, and hepatosplenomegaly. Whole-body computerized tomography showed enlarged lymph nodes in mediastinal, right axillary, abdominal para-aortic, ileum, and inguinal regions. Hepatosplenomegaly was also detected. In addition to abnormal liver function tests, serum levels of soluble interleukin-2 receptor and ferritin were elevated. Excisional biopsy of right axillary lymph node was performed in February 2002. Histological examination showed a diffuse proliferation of medium-to large-sized cells with round or oval nuclei and abundant cytoplasm. Spindle shape cells and Hodgkin-like giant cells were also seen. Immunohistochemically, the tumor cells expressed S-100 protein, CD 68, and CD 45 RO. They were negative for CD 1, CD 3, CD 15, CD 20, CD 21, CD 23, FDC, DRC, and p80. These findings were compatible with the diagnosis of IDCS. The patient was treated with polychemotherapy consisting of doxorubicin,cyclophosphamide, vincristine, and prednisone. However, he developed fungal pneumonia and died of respiratory failure 1 month after the start of treatment.