OBJECT: White matter diseases, including demyelinating or inflammatory disorders, may be indistinguishable clinically and radiologically from some central nervous system (CNS) tumors. In such situations, determination of the final diagnosis is difficult. An example is the differential diagnosis of non-acquired immunodeficiency syndrome-related primary central nervous system lymphoma (PCNSL) and multiple sclerosis (MS), a demyelinating disease. Unfortunately, delayed diagnosis and treatment of PCNSL can negatively affect prognosis. METHODS: The authors reviewed the cases of eight patients with PCNSL or MS. In each case, the initial diagnosis (PCNSL or MS) was equivocal. In these cases, conventional diagnostic approaches were not definitive, thus further delaying diagnosis. The initial symptoms, the selected diagnostic tests, and the presumptive as well as final diagnosis for each case are discussed. The final diagnosis was PCNSL in six cases and MS in two. The uncertainty about the clinical or initial pathological presentation required further diagnostic evaluation in all cases. Two important neurosurgical guidelines are the avoidance of corticosteroid agents and performance of biopsy sampling rather than volumetric tumor resection. Highvolume lumbar puncture, slit-lamp examination/vitrectomy, new CNS imaging techniques, and repeated biopsy procedures also proved helpful. CONCLUSIONS: In PCNSL, early definitive diagnosis and treatment are the keys to successful outcomes. Knowledge of strategies essential to early diagnosis lessens the need for brain biopsy sampling, but this procedure is still usually necessary. In such selected cases, biopsy sampling is appropriate even when pathological investigation shows MS rather than PCNSL. Complete resection is not indicated in PCNSL and can lead to additional sequelae.