Primary central nervous system lymphoma (PCNSL) is characterized by perivascular growth of large atypical lymphoid cells. With continued proliferation, the distribution becomes more diffuse and sheet-like. If the patient is given steroid therapy before biopsy, the tumor may shrink considerably and show prominent apoptosis and necrosis (as demonstrated here). The biopsy material may consist predominantly of reactive T-cell mimicking a chronic inflammatory process. Therefore, if PCNSL is suspected clinically, steroid therapy should not be prescribed before establishing the diagnosis.