Change of CD20 expression was examined in cases of diffuse large B-cell lymphoma (DLBCL). CD20 expression after treatment with anti-CD20 antibody (rituximab, Rx) for DLBCL was examined in 23 cases who received serial biopsy by immunohistochemistry (IHC) and flow cytometry (FCM). CD20- by IHC and/or FCM was defined as CD20-. Four cases were CD20- at initial biopsy but became CD20+ after chemotherapy with Rx (CH-R) (group A). Recurrent tumors in three group A cases became resistant to CH-R. Initial and recurrent tumors were CD20+ before and after CH-R in 17 cases (group B). Tumors before CH-R were CD20- in two cases (group C) and continued to be CD20- in one and turned CD20+ in the other with survival time after the relapse of 8 and 23 months, respectively. Evaluation of CD20 expression with immunohistochemical and flow cytometric methods is used for the prediction of responsiveness of relapsed DLBCL for CH-R.