CNS relapse occurs in about 5% of patients during the course of diffuse large B-cell lymphoma and entails a dismal prognosis. This consideration has led to the adoption of CNS prophylaxis, although known risk factors do not allow for an accurate prediction of CNS recurrences because they have insufficient sensitivity and specificity. Here, we review the reports of CNS events in major studies of diffuse large B-cell lymphoma before and after the introduction of rituximab, and probe the evidence that underlies prophylactic strategies such as intrathecal or high-dose intravenous chemotherapy. Now that rituximab is available, CNS prophylaxis relies on little-if any-evidence and should not be routinely administered. Nonetheless, several patient subgroups probably have a high risk of systemic and CNS relapses, and how to manage their treatment is a challenge. These subgroups include patients with testicular lymphoma or those who have more than one extranodal site involved plus at least one additional risk factor. For such patients, we recommend against prophylactic intrathecal chemotherapy because of the rare occurrence of isolated leptomeningeal relapses, the absence of evidence-based efficacy, and the potential harmful side-effects that are associated with this procedure. Because many CNS events are a result of primary resistance to treatment or accompany systemic relapses, high-dose intravenous methotrexate has been suggested as an alternative approach that needs to be validated in prospective controlled trials.