In a Danish population-based non-Hodgkin's lymphoma (NHL) registry (LYFO) representing a population of 2.7 million all new cases of NHL were registered from 1st January 1983 to 31st May 1994. Incidence data of primary malignant tumours of the brain and central nervous system in western Denmark for the period 1971-1990 have been obtained from the Danish Cancer Registry. During the approximate 11-year period 3124 new cases of NHL were registered. Of these, 1152 (37%) were extranodal and 48 were non-AIDS related primary central nervous system lymphomas (PCNSL) accounting for 4.2% of extranodal NHL and 1.5% of all NHL, respectively. The average annual incidence rate of non-AIDS related PCNSL during the period was 1.56 cases per million population (age range: 15-85 yrs, median: 62 yrs, M/F ratio: 1). In a 23-year period there was no trend towards an increasing incidence of non-AIDS related PCNSL in a well-defined population. PCNSL accounted for 1.7% of all primary malignant brain tumours. Incidence of primary malignant brain tumours was stable, except for age ranges over 70 years. Histologically, 85% were high grade, centroblastic diffuse (60%) and immunoblastic lymphoma (13%) (Kiel classification). No T-cell lymphomas were detected. Treatment included surgical resection, whole brain irradiation (WBRT) and chemotherapy. Median survival for those receiving either WBRT or WBRT and chemotherapy was eight months and 20 months, respectively (p = 0.78). Overall survival was 53%, 38% and 26% at one, two and five years. Cox-regression analysis identified only one factor having independent impact on survival in performance score > or = 2 (PCNSL p < 0.001, RR = 5.8).