The incidence of primary central nervous system lymphoma (PCNSL) has increased over the past two decades. The MR imaging appearance of PCNSL plays a central role in the initial diagnosis, management and follow-up of patients. The purpose of this study was to describe the presence and frequency of the pre-contrast T1 hyperintensity (T1h) that is sometimes identified in the region of enhancing neoplastic disease following treatment of PCNSL. We also explored possible causes for this phenomenon that, to the best of our knowledge, has not been previously described. The MR imaging and relevant medical records of 221 patients with pathologically confirmed PCNSL were retrospectively reviewed. Only patients with both treatment and follow-up imaging at our institution were eligible for inclusion in the study. Patients with evidence of post-procedural blood products (pre-contrast bright T1 lesions) prior to the initiation of therapy were excluded. Out of 221 patients, 119 met the eligibility criteria and were included in this investigation. Following treatment, 75 patients (63 %) developed pre-contrast T1h not attributable to blood products. All patients with this finding had been treated with methotrexate chemotherapy. The development of pre-contrast T1h following treatment for PCNSL is common. The hyperintense T1 signal in these patients may be caused by the biochemical response of tumor cells to treatment. To assess the prognostic significance of this novel finding, additional studies focusing on disease recurrence and patient survival are warranted.