A 25-year-old woman presented with infectious mononucleosis. Serological studies demonstrated elevated IgM titres to both cytomegalovirus (CMV) and Epstein-Barr virus (EBV). The role of each of these agents in infectious mononucleosis is reviewed, as are literature reports of co-infection by these two viruses. Both near-simultaneous infections and temporally remote sequential infections with acute CMV triggering an immunoreactivation of EBV are reported in the literature. We believe the current case is most consistent with the latter. Infectious mononucleosis is a common infection of childhood and young adulthood. Although a variety of agents may be associated with infectious mononucleosis, EBV is the most common etiology. We encountered a patient with serological findings that were suggestive of the simultaneous presence of two etiological agents of infectious mononucleosis: EBV and CMV. This prompted an inquiry into how commonly dual infections are encountered and their significance.