BACKGROUND: The role of Epstein-Barr virus (EBV) in idiopathic pulmonary fibrosis (IPF) is uncertain. A study was undertaken to detect the virus in IPF as well as to clarify the influence of EBV on the clinical features of the disease. METHODS: Twenty nine lung specimens were obtained from patients with IPF, as well as five specimens from patients with systemic sclerosis with pulmonary fibrosis (SSc) and 15 specimens from controls. EBV DNA and EBV latent membrane protein 1 (LMP1) were detected using the PCR method and immunohistochemical analysis, respectively. RESULTS: EBV DNA was detected in 24 of 25 patients with IPF (96%), in all five patients with SSc (100%), and in 10 of 14 controls (71%). The detection ratio was significantly higher in patients with IPF than in controls (p = 0.047, odds ratio (OR) = 9.60, 95% confidence interval (CI) 0.9 to 96.9). Immunohistochemical analysis revealed that cuboidal epithelial cells were positively stained with anti-LMP1 antibody in nine of the 29 lung specimens from IPF patients. In contrast, neither the patients with SSc nor the control subjects showed positive staining. In the follow up periods LMP1 positive patients with IPF died more frequently from respiratory failure than LMP1 negative patients (4/9 versus 1/20; p = 0.022, OR = 15.20, 95% CI 1. 3 to 168.0). CONCLUSIONS: EBV LMP1 positivity may be associated with more rapid disease progression in IPF.