To elucidate the relationship between Reed‐Sternberg (R‐S) cells and background T cells, the expression of CD80 and CD86 of R‐S cells in Hodgkin's disease (HD), and the ligand CD28 expression and the MIB‐1 index of background T cells were immunohistochemically investigated. CD80 and CD86 were found to be expressed on R‐S cells in almost all cases of HD. CD28 was expressed with strong intensity on many background T cells around R‐S cells. The MIB‐1 index of background T cells was 30.3% (range, 15.5–38.9%) and was much higher than 10.9% (range, 9.8–11.7%) in B cell lymphomas. These results suggest that the interaction between CD80 and CD86 on R‐S cells, and CD28 on background T cells may induce T cell proliferation and be associated with tumor mass of HD.