Parallel studies of (a) patients with Epstein-Barr virus (EBV)-associated peripheral T-cell proliferative disease/lymphomas and (b) a group of patients with a prolonged fever from other causes were conducted at Songklanagarind University Hospital from 1997 through 2000. (Reports on EBV-associated peripheral T-cell and NK-cell proliferative disease/lymphomas have been published elsewhere) In this study, the authors identified 58 patients; 14 were non-Hodgkin's lymphoma of B-cell origin (NHL-B), 8 were Hodgkin's disease, 6 were acute leukemia, 9 were systemic lupus erythematosus (SLE), and 21 were patients with other diseases. Serologic tests for the EBV infection, the study of EBV genome in circulating non-T-cells (CD3-cells) and T-cells (CD3+ cells), and the EBV-RNA study in the tumor cells were performed. EBV internal repeat-1 region (IR-1) in peripheral blood CD3+ cells was detected in 10 of 14 patients (71.5%) with NHL-B, 3 of 8 patients (37.5%) with Hodgkin's disease, 1 of 6 patients (16.7%) with acute leukemia, 4 of 9 patients (44.5%) with SLE, and was not detected in any of the 21 patients with other diseases. Anti-viral capsid antigen-IgG was significantly elevated in hematologic malignancy patients with EBV IR-1 genome in the peripheral blood CD3+ cells when compared to hematologic malignancy patients with a negative result, whereas there was no significant difference in anti-EBV nuclear antigen among these two groups. EBV-RNA expression in tumor cells by in situ hybridization was detected in 4 of 13 patients (31%) with NHL-B (all showed EBV IR-1 genome in peripheral blood CD3+ cells), and 3 of 5 patients (60%) with Hodgkin's disease (only two showed EBV IR-1 genome in peripheral blood CD3+ cells). These data support the theory that chronic EBV infection is often found in association with cases of NHL-B, Hodgkin's disease, acute leukemia, and SLE.