This review summarizes current knowledge of secondary central nervous system lymphoma (SCNSL) in adults. We define SCNSL as CNS involvement not obvious at the initiation of treatment for systemic lymphoma. Recently, polymerase chain reaction and flow cytometry assays of cerebrospinal fluid have become available for the correct diagnosis of SCNSL. We reviewed reports of patients treated without CNS prophylaxis to evaluate the incidence of SCNSL. Elevated serum lactate dehydrogenase levels, the involvement of more than one extranodal site, an advanced stage, a high age-adjusted International Prognostic Index score at presentation, and special anatomic sites of involvement such as the testis are important risk factors for SCNSL. Histologic evidence of aggressiveness is generally an indicator of risk for SCNSL. In addition to conventional treatment, stem cell transplantation, intrathecal administration of rituximab, and liposomal cytarabine have come into clinical use for the treatment of established SCNSL. Prevention of isolated CNS recurrence is thought to be the main target of CNS prophylaxis. The value of CNS prophylaxis according to histologic subtype, status of systemic lymphoma, and other risk factors is summarized. Although prophylaxis is fundamental for treating highly aggressive non-Hodgkin's lymphoma (NHL), it is beginning to be appreciated for the treatment of aggressive NHL. CNS involvement is almost always fatal; however, a CNS-active strategy could complement other approaches that have led to recent improvements in the prognosis for lymphoma.