Immunohistochemical detection of Epstein-Barr virus (EBV)-encoded latent membrane protein-1 (LMP-1) was used to identify EBV-associated Ki-1-positive anaplastic large-cell (ALC) lymphomas occurring in 11 patients with and 29 patients without human immunodeficiency virus (HIV) infection. In addition, 18 representative cases of other acquired immunodeficiency syndrome (AIDS)-related lymphomas and 66 cases of Hodgkin's disease, including 14 patients with HIV infection, were investigated. In patients with HIV infection, LMP-1 was found more frequently in Ki-1-positive ALC lymphomas than in other histotypes, although the difference in EBV association between Ki-1-positive ALC and other lymphomas was not significant. In these patients, the percentage of LMP-1 expressing Ki-1-positive ALC lymphomas was significantly higher than that found in patients without HIV infection (72.7% vs. 24.1%; P < .01), thus suggesting an etiologic role for EBV in a large proportion of AIDS-related Ki-1-positive ALC lymphomas. Moreover, the frequency of LMP-1 expression in Hodgkin's disease cases (71.4% in patients with and 21.1% in patients without HIV infection) was close to that found in Ki-1-positive ALC lymphoma cases, supporting the view that the higher frequency of EBV association with both entities detected in patients with HIV infection may be AIDS-related.