Unexplained liver test abnormalities are frequent in patients with Turner's syndrome. This cohort study was performed to clarify the histopathologic features, causes, and long-term outcome of liver involvement in these patients. Thirty patients with persistently abnormal liver test results were followed-up for 8.8 ؎ 5.2 years. Liver specimens were available in 27 patients. Marked architectural changes were present in 10 patients, including nodular regenerative hyperplasia in six, multiple focal nodular hyperplasia in two, and cirrhosis in two patients. These changes frequently were associated with obliterative portal venopathy lesions and with aortic malformations. There was mild to moderate portal fibrosis in 15 of the 17 other patients, inflammatory infiltrates in nine patients, and nonalcoholic fatty liver disease in 11 patients. Bile duct alterations resembling small duct sclerosing cholangitis were observed in 21 patients (with or without architectural changes). There was no viral, alcoholic, autoimmune, or drug-induced liver damage. Portal hypertension was observed in four patients with marked architectural changes, including three in whom refractory ascites or recurrent variceal bleeding developed, one of whom underwent transplantation. None of the patients without marked architectural changes experienced progressive or decompensated liver disease. There was no evidence of liver toxicity from estrogen replacement therapy. In conclusion, the main causes of liver involvement in Turner's syndrome are vascular disorders, probably of a congenital origin, and nonalcoholic fatty liver disease. In patients with vascular disorders, severe liver disease requiring liver transplantation may develop. Estrogen therapy does not appear to be pathogenically implicated. (HEPATOLOGY 2004;39: 239 -247.