We studied open-lung biopsies from 17 patients with pulmonary lymphomatoid granulomatosis (LYG) using paraffin-section immunostains and Epstein-Barr virus (EBV) RNA in situ hybridization to assess the phenotype of these unique tumors and to clarify the role of EBV infection. Histologically, all cases demonstrated the characteristic mixed mononuclear cell infiltrate of lymphomatoid granulomatosis with variable numbers of cytologically atypical large lymphoid cells in a background of small lymphocytes. Paraffin-section immunostains in all cases showed a predominance of T lymphocytes. A minor population of CD20-positive large B lymphocytes was identified in 11 cases; immunoglobulin light-chain restriction was demonstrated in four of these and immunoglobulin gene rearrangements in another case. Nuclear labelling for EBV RNA was detected in 10 of these 11 cases and was confined to the population of large B lymphocytes. Staining for CD20 was absent in the remaining six cases, as was nuclear labeling for EBV RNA. However, the large atypical lymphoid cells stained for T-cell-lineage-specific antibodies in three of these cases. We conclude that some cases of lymphomatoid granulomatosis are B-cell lymphoma associated with EBV infection, whereas others are of T-cell origin and are probably unrelated to EBV infection.