A 56-year-old previously-high-functioning man was evaluated by neurology service due to progressive memory loss for one month. On exam he recalled 2 of 3 objects in 3 minutes. Routine labs were all within normal range. A review of home medication showed no suspicion for any drug that could alter his cognition to this degree. Magnetic resonance imaging (MRI) revealed FLAIR hyperintensities within posterior periventricular white matter and posterior body of the corpus callosum with patchy enhancements. An electroencephalogram showed generalized background slowing. Cerebrospinal fluid (CSF) analysis showed lymphocytic pleocytosis, high protein and normal glucose. Broad spectrum antibiotics and acyclovir were started but discontinued after negative culture and two negative HSV PCR. The first and 2nd cytology showed no malignant cells while cytometry showed rare B cells with no atypical cells. Work-up including autoimmune and paraneoplastic panel, CSF 14-3-3, RT-QuIC, CT angiogram of head, CT of chest, abdomen and pelvis were unremarkable. Magnetic resonance spectroscopy (MRS) indicated neoplastic process. A 3rd CSF cytometry showed atypical cells. Diagnosis was finalized as LC after brain biopsy. Patient made a full recovery after chemo and stem cell transplant. His follow-up MRIs showed resolution of the pathological enhancements, though as of this writing there is no evidence of lymphoma recurrence.