Brain biopsy plays a crucial role in the exploration of suspect white matter lesions in the differential diagnosis of primary central nervous system lymphoma (PCNSL) and inflammatory demyelination. We present the case of a previously healthy, immunocompetent woman, aged fifty-nine, who developed a histologically confirmed demyelinating white matter lesion months prior to the manifestation of a PCNSL. Similar cases of “sentinel lesions” preceding a PCNSL have been reported. In a literature review, we compared the diagnostic features that may be useful to differentiate a PCNSL from inflammatory demyelinating disease in older age. We conclude that the occurrence of large, contrast-enhancing cerebral lesions in older patients with a relapsing-remitting disease course and steroid-resistant vision disorders should lead to the consideration of a PCNSL.