In a prospective study of 33 adults with portal vein thrombosis unrelated to a liver tumor, we have assessed the prevalence of primary myeloproliferative disorders using conventional criteria and cultures of bone marrow progenitor cells. A primary myeloproliferative disorder was documented in 14 patients investigated at the time of recognition of portal vein thrombosis. Among these 14 patients, the main clue to the presence of the myeloproliferative disorder was (a) the observation of suggestive abnormalities of peripheral blood cell counts in 4 patients; (b) characteristic findings at bone marrow biopsy or determination of total red cell volume in 3 patients; and (c) formation of "spontaneous" erythroid colonies in cultures of bone marrow progenitor cells in erythropoietin-poor medium in 7 patients. In 2 other patients, agnogenic myeloid metaplasia with myelosclerosis of apparently recent onset developed 5 yr after recognition of portal vein thrombosis. In conclusion, primary myeloproliferative disorders--in a full-blown or latent form, or at an early stage--are a major cause of portal vein thrombosis.