Thirty cases of mesenchymal hamartoma of the liver are presented and compared with 42 previously published cases. The patients, 69% male, range in age from newborn to 10 years (average age 15 months, median age 10 months). Except for occasional cases in which the lesion is an incidental finding at autopsy, most present with progressive abdominal enlargement over a period of days to months. Physical and radiologic examinations are nonspecific except for the demonstration of a mass within or attached to the liver. The masses are large, averaging 1311 g, and frequently contain cysts filled with clear fluid or gelatinous material. Histologic examination shows a mixture of loose mesenchymal tissue, bile ducts, connective tissue, and hepatocytes along with cysts formed either from degenerative areas of mesenchyme or from dilated bile ducts and lympathics. Treatment is partial or total excision of the lesion. Mortality (7-17%) is related to intraoperative or postoperative complications. Recurrence or malignant transformation has not been noted.