Most neurosurgeons and neurologists still consider lymphomatoid granulomatosis (LG) as a type of central nervous system (CNS) vasculitis. However, new insights, primarily from the hematologic literature, have cast doubt on the benign character of this disorder. In this article we (1) report a case of an 18-year-old woman with diffuse CNS disease and no mass lesion who developed multiple small cortical infarcts and dementia secondary to multifocal angiocentric, angiodestructive lymphoma; we (2) review other cases of LG with predominant CNS involvement; we (3) summarize the current understanding of LG, which is now considered to be a premalignant or overt angiocentric, angiodestructive T-cell lymphoma rather than a non-neoplastic vasculitis: the importance to neurologic surgeons and neurologists is that while pulmonary involvement in LG is generally the most prominent finding, patients may present with early or dramatic CNS disease; and we (4) note that although dementia is uncommon in young adults, this report adds yet another rare condition to the long differential list of dementia in this age group.