On the basis of earlier reports associating Epstein-Barr Virus (EBV) with half of the cases of idiopathic pulmonary fibrosis (IPF), we hypothesized that chronic infection with EBV or a closely related herpesvirus would be detected in all cases of IPF. We tested lung specimens from 33 IPF patients (8 patients with familial IPF and 25 patients with sporadic IPF) and 25 patients with other diseases as controls for the presence of eight herpesviruses using PCR-based techniques. One or more of four herpesviruses (cytomegalovirus [CMV], EBV, human herpesvirus 7 [HHV-7], and HHV-8) were detected in 32 of 33 (97%) subjects with IPF and in 9 of 25 (36%) controls (P < 0.0001). CMV, EBV, and HHV-8 were found more frequently in IPF patients than in controls (P < 0.05, P < 0.001, and P < 0.01 respectively). Two or more herpesviruses were detected in 19 of 33 (57%) IPF patients and in 2 of 25 (8%) controls (P < 0.001). Two or more herpesviruses and HHV-8 were found more frequently in patients with sporadic IPF than in patients with familial IPF (P < 0.05 for both comparisons), and CMV was found less frequently in patients with sporadic IPF than in patients with familial IPF (P < 0.05). Immunohistochemistry for EBV or HHV-8 antigen showed viral antigen primarily in airway epithelial cells. These data support the concept that a herpesvirus could be a source of chronic antigenic stimulation in IPF.