We examined histologically the bile duct lesions from 53 patients with end-stage primary sclerosing cholangitis (PSC) and compared them with similar lesions found in 25 surgically excised carcinomas of the extrahepatic bile ducts not associated with PSC. Of the 53 cases of PSC, 50 bile ducts were obtained at liver transplantation, two common bile ducts were segmentally resected for almost complete obstruction, and the entire extrahepatic biliary tract of another case was obtained at autopsy. Twenty bile ducts from patients who died without evidence of biliary tract disease served as controls. A modest increase in the number of intramural glands (mild hyperplasia) was noted in 13 cases (24.5%) of PSC. A marked increase in the number of intramural glands (florid hyperplasia) was found in 14 cases (26.4%) of PSC. In one case of florid hyperplasia, there was perineural and intraneural invasion of benign hyperplastic glands, which still maintained their lobular pattern. All cases of florid hyperplasia of intramural glands were accompanied by extensive fibrosis and marked nerve proliferation. Three of 24 (12.5%) invasive adenocarcinomas of the extrahepatic bile ducts showed mild hyperplasia of intramural glands without excessive nerve proliferation. Four invasive adenocarcinomas and one in situ carcinoma of the extrahepatic bile ducts showed florid hyperplasia of intramural glands (16%). The hyperplastic intramural glands were p53 negative and had low proliferative activity as measured by the low MIB-1 labeling index. In contrast, both in situ and invasive carcinoma expressed p53 protein and had a high MIB-1 labeling index. Focal high-grade dysplasia was found in one case of PSC (1.8%) and a small invasive adenocarcinoma in another (1.8%). Hyperplasia of intramural glands of the extrahepatic bile ducts is a reactive process that lacks specificity and is part of the morphologic spectrum of end-stage PSC. The incidence of dysplasia in PSC is low. Small invasive adenocarcinomas may be incidentally found in end-stage PSC, and detecting their presence before liver transplantation may be impossible.