A gradually enlarging axillary mass in a 79-year-old man was excised. The specimen was processed for light microscopy, immunohistochemical studies, and electron microscopy; gene rearrangement studies were also performed. A diagnosis of an interdigitating dendritic cell tumor of the lymph node and a B-cell small lymphocytic lymphoma occurring in the same anatomic location was made. We found that although rare cases of interdigitating dendritic cell tumor with an associated secondary malignancy have been described in the literature, to our knowledge, this is the first report of interdigitating dendritic cell tumor and synchronous neoplasm diagnosed at the same site. A possible relationship between the 2 disorders is also discussed.