Non-Hodgkin's lymphoma of the paranasal sinus is an uncommon presentation of extranodal lymphoma. Its natural history, treatment and prognosis have been infrequently characterized in the medical literature; however, a tendency to involve the central nervous system (CNS) has been noted. In British Columbia (population 4 million), a central database for lymphomas has allowed us to accurately track cases of paranasal sinus lymphoma diagnosed since 1980. A retrospective review was performed on the 44 patients who presented with primary paranasal sinus lymphoma (stage I or II) between 1980 and 1999. Histologic features were identified and immunophenotypic classification performed. Complete diagnostic and follow-up data including stage, treatment, response rates, sites of relapse and survival data were available for all patients. There were 26 men and 18 women. The types of lymphoma found were: diffuse large B cell (including immunoblastic), n = 37 (84%); T/NK nasal type, n = 3 (8%); peripheral T cell, not otherwise classified, n = 2 (4%); and others, n = 2 (4%). The median age at presentation was 66 years (range 27-97 years). The median follow-up for living patients was 114 months. For all 44 patients, the 5- and 10-year overall survivals were 48% and 41% and the disease-specific survivals 62% and 62%, respectively. Beginning in May 1985, intrathecal chemotherapy was added to our standard treatment plan of multi-agent chemotherapy and local irradiation. Before 1985, 2 of 5 patients developed leptomeningeal metastasis. Following the institution of intrathecal chemotherapy, only 8% (3 of 39) of patients have developed CNS disease. Introduction of intrathecal chemoprophylaxis was also associated with an improvement in overall survival from 20% to 51% and disease-specific survival from 40% to 65%. Primary paranasal sinus lymphoma is an uncommon presentation of lymphoma that carries the potential risk of spreading to the leptomeninges. Treatment with combined modality chemotherapy and irradiation can cure many patients and the addition of intrathecal chemotherapy may reduce the risk of CNS relapse.