This study comprises the 255 adult AIDS patients treated at Ullevål hospital 1983-1995. These patients, fulfilling the Centers of Disease Control (CDC) clinical criteria for AIDS, correspond to 91% of all adult AIDS cases in Oslo. By the end of the study period, 44 patients were alive and 211 had died. Full autopsy was performed on 153 (73%) of the deceased. Supplementary analyses were carried out on the 344 patients (225 deceased) fulfilling the U.S. definition of AIDS, which includes CD4 cell counts below 200 cells/mm3. In the autopsy group, histologically verified non-Hodgkin B-cell lymphoma was found in 29 cases (19%). Nineteen of these (12%) had primary central nervous system (PCNS) lymphoma. Survival curves indicate that PCNS lymphoma constitutes a small risk early in the AIDS stage, but it has a serious impact on long-term survival. For patients not contracting other fatal diseases, one fourth are estimated to die of PCNS lymphoma within about 3 years. Comparison of clinical diagnoses and autopsy results show that PCNS lymphoma has been difficult to separate from other CNS disorders, which probably has caused marked underestimation of the incidence in previous assessments. We conclude that PCNS lymphoma is a major threat to long-term survival in AIDS victims.