Rituximab is a chimeric anti-CD20 monoclonal antibody. It has shown efficacy in patients with B-cell non-Hodgkin lymphoma and also in CD20-positive Hodgkin lymphoma. Recently, CD20-negative tumors have been described after Rituximab therapy. We report a 34-year-old man with a history of nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL), treated with different chemotherapy regimens, including anthracyclines and Rituximab. After 4 years in complete remission, he developed a CD20-negative T-cell-rich B-cell lymphoma (TCRBCL) presenting as multiple lung lesions. This case shows the difficulties in the diagnosis of CD20-negative lymphomas when the number of tumor cells is low and when they are found in a predominant T-cell context. Using anti-CD79a as a B-cell marker is mandatory to overcome the difficulties in identifying these tumors. Moreover, this case illustrates the usefulness of laser capture microdissection to obtain purified cell populations for molecular studies in lymphomas with relative paucity of tumor cells, as well as the need to analyze different IgH gene regions to decrease the rate of false-negative results in PCR clonality studies.