An increased hematocrit can be caused by primary proliferative polycythemia (PPP), by secondary polycythemia, by relative polycythemia (reduced plasma volume with a normal red cell mass), or by modifications of the red cell mass and the plasma volume within their normal ranges. As an increased hematocrit by itself is a risk factor for thrombosis, it is important to diagnose not only polycythemia, but also its possible cause, in order to offer optimal therapy. Smoking is the most frequent cause of an increased hematocrit. Splenomegaly, aquagenic pruritus, and erythromelalgia often exist in PPP, whereas other symptoms such as dyspnea are more likely to be associated with secondary polycythemia. Smokers with an increased hematocrit will be asked to stop smoking before ordering blood volume studies. These studies are not indicated in patients with obvious pulmonary disease. Male patients with an hematocrit over 60% and female patients with an hematocrit over 55% always have absolute polycythemia. The associations of an increased hematocrit with splenomegaly, a raised white blood cell count or thrombocytosis are indicators for PPP. The necessity for blood volume studies is questionable in these patients. However, blood volume studies are useful in patients with an increased hematocrit and no other clinical or biological signs suggestive of any form of polycythemia.