Primary central nervous system lymphoma (PCNSL) is a rare tumor. It occurs mainly in people aged 50 year-old or older and is more common among men. Immunodeficiency is the only established risk factor for PCNSL. We report a 35 year-old, immunocompetent woman who presented with a two weeks history of persistent headache. Computed tomography (CT) and magnetic resonance imaging (MRI) showed an expansive lesion in the right thalamus area. Immunohistochemical studies were consistent with the diagnosis of a difuse large B-cell non-Hodgkin 's lymphoma. The patient was treated with chemotherapy and whole brain radiotherapy, achieving complete remission of the tumor. This case is ilustrative of PCNSL and contributes to update its diagnosis, management and prognosis