Expression of the interleukin (IL)-10/BCRF1 gene was studied by in situ hybridization in tissue samples from acquired immunodeficiency syndrome (AIDS) lymphomas using a BCRF1 probe which also recognizes the human IL-10 sequence. Hybridization was detected in 8 out of 15 lymphomas. In contrast, the IL-10/BCRF1 gene expression was detected in only 1 out of 11 lymphomas from human immunodeficiency virus (HIV)-seronegative patients (p = 0.05). In AIDS lymphomas, the number of cells labeled with a BCRF1-specific probe was dramatically lower than that of cells labeled with the IL-10/BCRF1 probe. Thus, the IL-10 rather than the BCRF1 gene was expressed. Production of IL-10 was associated with that of IL-10 mRNA, as shown by immunodetection of the protein in numerous cells. In contrast, BCRF1-producing cells were rarely detected. Both in situ hybridization and immunochemical experiments indicated that malignant cells were involved in this IL-10 synthesis. IL-10 production in AIDS lymphomas was associated with the presence of Epstein-Barr virus (EBV) in lymphomatous cells (p = 0.02). As IL-10 is a potent growth factor for human B lymphocytes, these results suggest that IL-10 may stimulate the proliferation of malignant cells in an autocrine pathway in a number of AIDS lymphomas, and that EBV and HIV may synergistically trigger its production.