13 patients with cryptogenic fibrosing alveolitis (CFA) and 12 with interstitial lung disease (ILD) of known cause were studied for their humoral response to herpes simplex virus (HSV), cytomegalovirus (CMV), and Epstein-Barr virus (EBV). Serum antibodies to HSV and CMV were within the normal range in all patients. 10 patients with CFA had raised serum antibodies to EBV, and IgA against viral-capsid antigen (VCA) was detectable in all 13. In the other 12 patients EBV serological profiles were normal and IgA against VCA was detectable in only 1 patient. The EBV antibody levels did not correlate with the level of circulating immune complexes, the presence of rheumatoid factors, or the cytological findings of the alveolitis. The presence of IgG against VCA in 5 CFA patients suggests local production of EBV-specific immunoglobulins. Elevated IgG and IgA against EBV in CFA may indicate non-specific depression of cell-mediated immunity or that EBV plays a part in the aetiology of CFA.