Primary sclerosing cholangitis (PSC) is an uncommon cholestatic liver disease characterized by the inflammation and fibrosis of intra- and extra-hepatic biliary ducts. It has an incidence of 0.9 to 1.3 cases per 100000 in the United States. After recruiting of endoscopic retrograde cholangiopancreatography (ERCP) in the early 1970s, it revealed that 'classical' primary sclerosing cholangitis (PSC) appears as multiple strictures in the intra- and extra-hepatic biliary tree. However, the ERCP is not diagnostic for the detection of the small intra-hepatic biliary strictures. Then, in 1985, this question raised by Ludwig that whether the small bile ducts might be involved in PSC. Pathological studies in 1991 led to the coining of the term small duct PSC (SDPSC). Later, SDPSC was defined in the patients with inflammatory bowel disease and chronic cholestasis, who have normal cholangiogram with liver biopsy diagnostic for PSC.