We present a case of classical Hodgkin's lymphoma (HL) co-occurring with histological features of Castleman's disease (CD). A 25-year-old man presented with left supraclavicular and axillary lymph node swelling and mediastinal mass. Using an initial biopsy specimen from left axillary lymph node, a tentative diagnosis of multicentric CD of plasma cell type was made. The serum level of interleukin-6 (IL-6) was elevated. The patient was treated with immunosuppressive therapy containing tocilizumab (TCZ). Shrinkage of mediastinal mass and axillary lymph nodes was seen; however, swelling of his left axillary lymph nodes reemerged, even after therapy with TCZ. A second left axillary lymph node biopsy was performed and a diagnosis of nodular sclerosis of classical HL without histologic features of CD was made. The initial biopsy specimen was re-examined, and scattered CD30+ Hodgkin/Reed-Sternberg cells were found in the interfollicular area. Interestingly, Hodgkin/Reed-Sternberg cells and surrounding reactive cells in both lymph nodes were stained with anti-IL-6 antibody. We emphasize that biopsy specimens with HL involvement may also have histologic features reminiscent of those seen in CD. To our knowledge, this is the first report to provide a detailed description of this pathology, including a survey of IL-6 and clinical course upon treatment with TCZ.