In vitro colony culture and chromosomal studies in hepatic and portal vein thrombosis--possible evidence of an occult myeloproliferative state. We have studied the prevalence of an underlying myeloproliferative state in 20 patients with either hepatic or portal vein thrombosis. Using conventional clinical and laboratory criteria, an underlying myeloproliferative state was identified as the cause of the thrombosis in five patients (25%). A further 10 of the remaining 15 cases were found to have characteristic in vitro bone marrow culture studies and cytogenetic abnormalities suggestive of an underlying myeloproliferative disorder. Although none of these 10 cases have developed overt clinical and laboratory features of such a myeloproliferative disorder after a median observation period of two years, the presence of clonal karyotypic abnormalities in three cases, increased megakaryocyte colony growth in three cases and endogenous erythropoietin independent colony growth of the marrow erythroid progenitors in seven cases, argues strongly in favour of a primary haematological disorder. This has important therapeutic implications, particularly in cases being considered for orthotopic liver transplantation. 