IgG4-related lymphadenopathy with increased numbers of Epstein-Barr virus (EBV)-infected cells has been reported but not fully described. We analyzed 31 cases of IgG4-related lymphadenopathy and 24 cases of extranodal IgG4-related diseases for their possible relationship with EBV. Other types of reactive lymph nodes (22) and angioimmunoblastic T-cell lymphoma (AITL) (10) were also studied for comparison. EBV-encoded RNA (EBER) in situ hybridization revealed EBER(+) cells in 18 of 31 cases (58%) of IgG4-related lymphadenopathy. Increased EBER(+) cells were found in only 4 of 22 (18.1%) non-IgG4-related reactive lymphoid hyperplasia in patients of a similar age (P=0.002) and in only 5 of 24 (21%) extranodal IgG4-related biopsies (P=0.006). Interestingly, all patients with EBER(+) progressively transformed germinal center-type IgG4-related lymphadenopathy had systemic lymphadenopathy and/or extranodal involvement. AITL also is associated with EBV, and IgG4-related lymphadenopathy sometimes mimics the morphology of AITL; however, the number of IgG4(+) cells in AITL was significantly less than that in IgG4-related lymphadenopathy (P<0.001). Increased numbers of regulatory T cells are seen in IgG4-related disease; however, there was not a significant difference between the EBER(+) and EBER(-) cases. In conclusion, the presence of increased numbers of EBV-infected cells in IgG4-related lymphadenopathy, compared with other reactive lymphadenopathy or extranodal IgG4-related disease, suggests that there may be a relationship at least between nodal IgG4-related disease and EBV. It is important to avoid overdiagnosing these cases as malignant lymphomas or EBV-related lymphoproliferative disorders.