An attempt was made to discover the aetiology of acquired portal vein thrombosis in 12 polycythaemic patients who did not show any obvious local or regional cause. In addition to the diagnostic criteria of polycythaemia vera, erythropoietin was determined and cultures of erythroblast precursors were examined. The patients could be divided into 3 groups, in the first of which the definite diagnosis of polycythaemia vera was made on the basis of the PVSG (Polycythaemia Vera Study Group) criteria and bone marrow biopsy (5 patients). In the second group (5 patients), there was a diagnosis of possible polycythaemia vera based essentially on the finding of a spontaneous growth of medullary CFU-E. Finally, diagnostic criteria for polycythaemia vera were absent in two patients. On the basis of these findings, the physiopathology of the association of portal thrombosis and polycythaemia is discussed, in particular polycythaemia secondary to hepatic ischaemia.