CD26/dipeptidyl peptidase IV is a cell surface antigen with multiple biological functions. Although its involvement in tumor biology has been suggested, the significance of its expression in malignant lymphoma has not been clarified in detail. This study examined the expression of CD26 and cell surface adenosine deaminase (ADA) in 42 cases of Hodgkin's lymphoma (HL) and T-cell lymphoma by immunohistochemistry on frozen sections. CD26 was expressed in three of 14 cases of HL, in four of eight cases of anaplastic large cell lymphoma (ALCL), in two of nine cases of peripheral T-cell lymphoma, in one of six cases of lymphoblastic lymphoma and in none of three cases of adult T-cell lymphoma/leukemia. Expression of cell surface ADA was fully correlated with the expression of CD26 and expression of CD26/ADA in ALCL and HL was also completely correlated with the expression of p80 and epithelial membrane antigen. Of 10 CD26-positive patients, seven had fever and elevated CRP at initial diagnosis and over a median follow-up of 61 months (range, 7 - 152 months) only three survived. This study suggested that CD26 is selectively expressed on ALK-positive, but not on ALK-negative, ALCL and HL. This is also the first report to demonstrate that ADA is coexpressed with CD26 on the cell surface of malignant neoplasms in vivo.