We measured rituximab concentrations in the cerebrospinal fluid (CSF) in 2 patients with diffuse large B-cell lymphoma in whom central nervous system (CNS) invasion had developed. They received rituximab intravenously following irradiation therapy (patient no.1) or along with chemotherapy (patient no.2). The rituximab concentrations in the CSF were considered to be very low (0.2-0.6 microg/ml), and could not be increased significantly by serial intravenous administrations of rituximab. The lymphoma relapsed in patient no.1, and the combined therapy was not effective in patient no.2. An alternative approach such as intrathecal administration of rituximab could be anticipated as a new therapeutic strategy for CNS lymphoma.