We studied 14 adult patients presenting with fever and cytopenia of the peripheral blood and histiocytic hyperplasia with hemophagocytosis (HHH) in the bone marrow regarding an association of cytomegalovirus (CMV) and Epstein-Barr virus (EBV) by using in situ hybridization (ISH) and also evaluated the clinical and laboratory findings according to the encountered organisms. ISH using a CMV RNA probe demonstrated infected cells in 6 out of 14 cases (43%), and ISH using an EBV EBER RNA probe demonstrated infected nuclei in 5 out of the same 14 cases (36%) of HHH. No cases showed a positive reaction with both probes. Three cases showed a negative reaction with both probes. The mean age of all patients was 29 years; and that of the CMV-positive patients was 27 years and that of the EBV-positive patients was 36 years. Organomegaly was found in 3 out of 6 CMV-positive patients (1 hepatomegaly, 1 splenomegaly, 1 hepatosplenomegaly), and 4 out of 5 EBV-positive patients (lymphadenopathy in all 4 cases, hepatosplenomegaly in 2 cases). One of the CMV-positive case had acute myeloblastic leukemia, and 2 EBV-positive cases had underlying malignancy (1 Hodgkin's disease, 1 non-Hodgkin's lymphoma). Seven out of the 14 HHH cases (50%) died within several months after diagnosis. Nucleic acid hybridization methods can be used for the routine examination of the association of CMV or EBV.