Primary sclerosing cholangitis and primary biliary cirrhosis are chronic cholestatic syndromes that may be difficult to differentiate clinically. Destructive cholangitis occurs in both diseases and leads to similar clinical and biochemical abnormalities. Therefore, we compared the clinical, biochemical, immunologic, radiologic, and hepatic histologic features of these syndromes in two large groups of patients prospectively selected by predefined criteria. Primary biliary cirrhosis (n = 258) occurred predominantly in middle-aged women who were usually symptomatic with fatigue and pruritus, commonly had keratoconjunctivitis sicca, and often were hyperpigmented. Tests for antimitochondrial antibodies were always positive, usually in very high titer. Although the extrahepatic bile ducts were normal radiographically, smooth tapering and narrowing of the intrahepatic bile ducts was occasionally noted. Hepatic histology was diagnostic when a florid duct lesion was present. In contrast, primary sclerosing cholangitis (n = 60) occurred primarily in young men who were usually symptomatic with fatigue and pruritus and frequently had chronic ulcerative colitis. Tests for antimitochondrial antibodies were nearly always negative and cholangiography demonstrated abnormalities of the extrahepatic and intrahepatic bile ducts in all cases. Although hepatic histology was often compatible with the diagnosis, it was usually not diagnostic, and considerable overlap existed with the abnormalities seen in primary biliary cirrhosis. Likewise, biochemical tests of copper metabolism were similar in both syndromes. These results call attention to the differences and similarities in the clinicopathologic features of these two cholestatic syndromes and provide a basis for a rational diagnostic strategy.