Mastocytosis is a disease of mast cell hyperplasia that may involve several organ systems, including liver. Between 1988 and 1991, we conducted a retrospective-prospective study of 41 patients with mastocytosis and found 61% had evidence of liver disease. Hepatomegaly was detected in 24%, splenomegaly in 41%, and elevated serum alkaline phosphatase, serum aminotransaminases, 5′nucleotidase, or gamma-glutamyltranspeptidase (GGTP) in 54% of the patients. Alkaline phosphatase levels directly correlated with GGTP levels, hepatomegaly, splenomegaly, and liver mast cell infiltration and fibrosis. Elevated alkaline phosphatase levels and splenomegaly were observed more frequently in patients with categories II and III mastocytosis. Five patients in combined disease categories II or III developed ascites or portal hypertension and died of complications of mastocytosis; three had hypoprothrombinemia at the time of death. Thirty-five liver biopsy specimens from 25 patients were examined. Mast cell infiltration was commonly observed in the biopsy specimens, more severe in those patients with either category II or III disease, and correlated with hepatomegaly, splenomegaly, alkaline phosphatase levels, and GGTP levels. Mast cells were often only detected by using special stains (toluidine blue and chloracetate esterase). Increased portal fibrosis was seen in 68% of the biopsy specimens and correlated with mast cell infiltration and portal inflammation. Cirrhosis was not observed. Nodular regenerative hyperplasia, portal venopathy, and venoocclusive disease was observed in eight biopsy specimens and may have been the cause of the portal hypertension or ascites in four patients. These findings demonstrate that liver disease with mast cell infiltration is a common finding in patients with mastocytosis. Severe liver disease, however, is uncommon except in patients with mastocytosis who are in categories II and III, where it may contribute to both morbidity and mortality.