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Objectives: Infiltration of many IgG4-positive plasma cells (G4-Ps) is seen in IgG4-related diseases and in several "non-IgG4-related diseases," such as pilonidal sinus (PS) as well. The involvement of CD4+CD25+ regulatory T cells (CD4CD25 Tregs) in IgG4-related diseases has been reported. To see whether CD4+CD25+ Tregs are involved in autoimmune pancreatitis (AIP)/non-IgG4-related diseases with many G4-Ps, we investigated the amount of G4-Ps and CD4+CD25+ Tregs histologically in AIP/PS., Methods: Four AIP and 10 PS were immunostained with IgG4/Foxp3, a specific marker for CD4+CD25+ Tregs. Double immunohistochemistry and dual fluorescent immunohistochemistry were conducted to see the amount of CD4+CD25+ Tregs., Results: All AIP and 30% of PS showed abundant G4-Ps. G4-Ps infiltrated diffusely for all AIPs and in a patchy pattern for PS at the abscess/granulation foci. Foxp3 immunostaining/double immunohistochemistry showed moderate to abundant CD4+CD25+ Tregs in AIP and abscess of PS, but few to moderate in granulation of PS. Dual fluorescent immunohistochemistry also showed many CD4+CD25+ Tregs in AIP., Conclusions: Many CD4+CD25+ Tregs were seen in AIP lesions, abscess of PS, but not in granulation of PS, suggesting that the amount of CD4+CD25+ Tregs sometimes do not synchronize with that of G4-Ps and might relate to the inflammatory activity of both AIP and PS.