This case report describes a patient with an abdominal sepsis and a CT scan showing portal vein thrombosis, splenic vein thrombosis and splenomegaly. Laparotomy findings include 1 metre of ischaemic small bowel, which is resected. Laboratory measurements show erythrocytosis, leukocytosis and thrombocytosis. This combination of findings is suspect for the diagnosis of one of the myeloproliferative disorders: essential thrombocythemia, primary myelofibrosis, chronic leukaemia, or polycythemia vera (PV). PV is a chronic myeloproliferative disorder characterised by erythrocytosis which results in hyperviscosity of the blood. The hyperviscosity may play a significant role in thrombosis. If left untreated, median survival is two years. Of the patients with PV, 95-99% have a point mutation of the JAK2 gene. The major treatment goal is the prevention of thrombosis by haematocrit control. In order to achieve this, phlebotomy in combination with a myelosuppressive agent, such as hydroxyurea, is recommended in patients with a high risk of thrombosis. It is crucial to make the diagnosis of PV, since treatment can prevent further thrombotic complications and increase survival.