Sclerosing cholangitis can be primary (PSC) or secondary. One unusual cause of secondary sclerosing cholangitis is the newly recognized entity of IgG4-associated cholangitis. The prevalence and significance of IgG4 plasma cells in patients, who are clinically and radiologically classified as PSC, however, are unknown. Clinical information and histology of liver explants of 98 consecutive liver transplants performed for PSC were reviewed. IgG4 immunohistochemical stain was performed on sections from hilar areas that contained large bile ducts and corresponding cholecystectomy specimens (available in 74 cases). Serum IgG4 levels were measured in stored serum from 81 cases. Tissue IgG4 positivity (>or=10 IgG4+ plasma cells/high power field) was correlated with clinical features (age, sex, presence of inflammatory bowel disease and cholangiocarcinoma, pancreatogram, PSC duration, PSC recurrence after transplant, and number of acute rejection episodes) and histologic findings (periductal lymphoplasmacytic infiltrate, storiform fibrosis, and obliterative phlebitis) in the liver explants. Twenty-three (23%) liver explants showed periductal infiltration with IgG4+ plasma cells. Eighteen cases (22%) had elevated serum IgG4 levels, including 8 without tissue IgG4 positivity. All cases showed dense periductal fibrosis; none had storiform fibrosis or obliterative phlebitis. IgG4 positivity in the liver strongly correlated with moderate-to-marked periductal lymphoplasmacytic inflammation (P=0.002). Clinically, IgG4 positivity in tissue, but not in serum, was correlated with shorter PSC duration before transplant and higher risk of recurrence after transplant. Nearly one quarter of explanted livers that carry a clinical diagnosis of PSC contain increased IgG4+ periductal plasma cell infiltrates and positive serum IgG4 levels. However, none of the explants show histologic features diagnostic of IgG4-associated cholangitis. PSC with tissue IgG4 positivity has a more aggressive clinical course manifested by shorter time to transplant and a higher likelihood of recurrence than IgG4 negative PSC.