Prior corticosteroid therapy presents a major challenge in the diagnosis of CNS lymphomas, particularly in stereotactic biopsies. In this study we analysed the cytological, histopathological and immunohistochemical features in stereotactic biopsies of 25 primary CNS lymphoma cases pre-treated with corticosteroids. We documented the extent and the frequency of each finding. We also investigated the significance of subjectivity in evaluation of these biopsies in 3 seperate sessions including the final diagnostic decision. In 48% of our cases the diagnosis was straightforward. These cases were characterized by prominent blasts either in diffuse paranchymal infiltrates or in perivascular regions. The remaining 52% demonstrated some degree of variability among pathologists. Lymphoid atypia other than the typical blastic morphology appeared as a subjective finding and this was more pronounced in cytology preparations. In our study, corticosteroid pre-treatment in primary CNS lymphoma was associated with a large spectrum of histopathological, immunohistochemical and cytological findings. Combined use of an extended immunohistochemical panel would increase the possibility of conclusive diagnosis. Nevertheless some of these findings and therefore the diagnosis are open to subjectivity.