Human herpesvirus (HHV)-6 and HHV-7 are increasingly being recognized as emerging pathogens among transplant recipients. Using quantitative polymerase chain reaction assays, we demonstrate the presence of HHV-6 and/or HHV-7 in 18 of 20 episodes of clinically presumed or microbiologically confirmed cytomegalovirus (CMV) infection. Seventeen (89%) of 19 microbiologically confirmed cytomegalovirus (CMV)-infected patients had concomitant HHV-6 variant B (47%) and/or HHV-7 (63%) infection. The degree of HHV-6 coinfection was significantly correlated with hyperbilirubinemia while HHV-7 coinfection demonstrated a non-significant trend toward cytopenias. In one of the 20 episodes described herein, the 'viral syndrome' was due solely to HHV-7 infection; clinical and virological response was observed during intravenous ganciclovir therapy in this patient. While this study emphasizes the significance of HHV-6 and/or HHV-7 coinfection during episodes of CMV infection, it significantly highlights the novel observation of the causal role of HHV-7 (in the absence of HHV-6 and CMV) in a clinical illness presumed to be caused CMV. Thus, HHV-7 (and HHV-6) should be considered as a pathogen (or copathogen) in the viral syndromes following organ transplantation.