The virus-associated hemophagocytic syndrome (VAHS) is a histiocytic proliferative disorder with bone marrow and liver failure for which the connection with a specific virus is often tenuous. Epstein-Barr virus (EBV) is one candidate for the association, but serologic or culture confirmation may be lacking in a particular case. As a means of directly identifying the presence of EBV in patients' cells, molecular hybridization studies were carried out using a radioactively labeled viral DNA segment. DNA from mononuclear cells of two children with VAHS had specific hybridization to the EBV DNA probe. One of the patients had serologic evidence for EBV infection. Several immunologic deficiencies were found. VAHS may represent one of several hematologic and/or immunologic dysfunctions caused by EBV.