Budd-Chiari syndrome is a severe disease characterized by occlusion of large hepatic veins leading to death if untreated. Using the classical criteria for the diagnosis of polycythemia vera (PV), essential thrombocythemia (ET), and idiopathic myelofibrosis (IMF), overt PV was the underlying cause in about 10% of the cases and ET or IMF in only a very few. Using spontaneous endogenous erythroid colony (EEC) formation in vitro and/or bone marrow biopsies, a primary myeloproliferative disorder (PMD) was present in 78% of the patients with apparently idiopathic Budd-Chiari syndrome and in about half of the patients with portal, splenic, and/or mesenteric vein thrombosis. The diagnoses in 40 reported cases with hepatic vein thrombosis and spontaneous EEC were overt PV in 25 and latent unclassified PMD in 15 patients. The diagnoses of 40 reported cases with splanchnic vein thrombosis and spontaneous EEC were overt PV in 12, ET in 2, IMF in 2, and latent unclassified PMD with the presence of EEC in 24 patients. Thrombocytosis as a manifestation of myeloproliferative disease was recorded in 34 of 80 (42.5%) patients with spontaneous EEC and Budd-Chiari syndrome or portal vein thrombosis. Thrombocythemia was present in 15 of 41 patients with a proven and in 19 of 39 patients with a latent myeloproliferative disorder. Patients with hepatic vein or splanchnic vein thrombosis associated with a PMD are predominantly females younger than 45. It is concluded that both spontaneous EEC and histopathology from bone marrow biopsy provide specific information as sensitive clues to the diagnosis of all variants of overt and latent myeloproliferative disorders. The association of hepatic and splanchnic vein thrombosis and PMD is not fully understood. Therapeutic options of Budd-Chiari syndrome include anticoagulation with heparin, fibrinolysis followed by oral anticoagulation, and appropriate treatment of the underlying PMD. In case of failure, invasive options include local procedures such as angioplasty or stenting, venous decompression by portal-systemic shunts, or liver transplantation.