Serological diagnosis of herpes virus infections is hampered by concurrent expression of IgM for heterologous members of this virus family. To assess the frequency of such multiple diagnostic findings and to understand their etiology, we sought by using IgG, IgM, and IgG avidity test serodiagnoses for Epstein-Barr virus (EBV) among immunocompetent or immune-suppressed patients with well-documented cytomegalovirus (CMV) primary infection. Controls had primary infection by EBV or had acute septic or severe respiratory infection. Among EBVseropositive patients with CMV primary infection, a large proportion (13/56, 23%) showed antibody profiles of EBV reactivation: seroconversion of VCA IgM and/or ΰ΄ fourfold rise of VCA IgG, together with high or intermediate avidity of VCA IgG. Most of the CMV patients with EBV serodiagnosis showed also diagnostic HHV-6 antibody rises. In contrast to the frequently occurring CMV-induced EBV immunoreactivation, EBV primary infections did not appear to induce immunoreactivations of CMV (0/22). Only one (2%) CMV patient had a significant varicella zoster virus (VZV) antibody rise. The studies show that CMV is a particularly active inducer of some, but not all, members of the herpes virus family and suggest that the in vivo interplay between CMV and EBV occurs unidirectionally. The high frequency of heterologous herpes virus immunoreactivations poses demands on laboratory diagnosis.