Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) is a low-grade B-cell neoplasm and ∼2% to 9% patients develop an aggressive lymphoma, most commonly diffuse large B-cell lymphoma (Richter transformation, DLBCL-RT). Programmed death-1 (PD-1) pathway plays a crucial role in tumor host immunity evasion and its blockade has emerged as an effective anti-cancer immunotherapy. PD-L1 and PD-1 expression has shown predictive value in anti-PD cancer immunotherapy; however, it has not been well documented in CLL/SLL and DLBCL-RT. We evaluated PD-1 and PD-L1 expression by immunohistochemistry in 39 CLL/SLL, 15 DLBCL-RT, and 26 other DLBCL. In CLL/SLL, neoplastic B-cell PD-1 expression was weak and restricted to prolymphocytes/paraimmunoblasts within proliferation centers (PCs) and accentuated PCs of all sizes. Neoplastic B-cell PD-1 expression was highly prevalent and demonstrated increased intensity in DLBCL-RT, but in contrast was only rarely seen in other DLBCL (12/15 vs. 1/26; P<0.0001). An excellent correlation (90% concordance) was observed between neoplastic B-cell PD-1 immunohistochemistry positivity and molecularly defined CLL/SLL clonal relatedness in DLBCL-RT. PD-L1 expression was observed on the neoplastic B cells in rare DLBCL-RT and other DLBCL cases (1/15 vs. 1/26; P>0.05) as well as background histiocytes and dendritic cells. Overall survival of DLBCL-RT was significantly inferior to that of the other DLBCL (median, 16.9 vs. 106.1 mo; P=0.002). Our findings suggest a biological continuum from prolymphocytes/paraimmunoblasts in CLL/SLL PCs to the neoplastic B-cells in DLBCL-RT. The characteristic PD-1 expression in DLBCL-RT makes it a potential surrogate marker for determining clonal relatedness to CLL/SLL, which may have important prognostic and therapeutic implications.