We assessed the prevalence of overt and latent primary myeloproliferative disorders in hepatic vein thrombosis. Cultures of bone marrow or peripheral blood mononuclear cells were done in 20 patients with Budd-Chiari syndrome. Erythroid colony formation in the absence of erythropoietin, which is a reliable indicator for a primary myeloproliferative disorder, was seen in 16 patients in whom Budd-Chiari syndrome was due to hepatic vein thrombosis, including 13 women aged 18 to 45 years. Among these 16 patients, the conventional criteria for the diagnosis of a primary myeloproliferative disorder were met in only 2. Primary myeloproliferative disorder, often without peripheral blood changes, is a major cause of hepatic vein thrombosis in young women.