Primary central nervous system lymphoma (PCNSL) is a rare neoplasm that can affect the brain, eyes, and, rarely, the spinal cord. Clinical presentation and MRI findings can mimic a variety of diseases, including high-grade gliomas, infectious and granulomatous diseases, and demyelinating diseases. We describe three cases where the diagnosis of PCNSL was difficult due to an ambiguous clinical, radiological and laboratory results. The role of stereotactic biopsy remains leading in differential diagnosis; however, the invasiveness and frequent limitations of this method determine the search for additional biological markers of the disease. New evidence suggests a potential role for cerebrospinal fluid (CSF) cytokine profiles and proteomic analysis in differential diagnosis, disease progression, and treatment response.