Thirty four consecutive patients (31 male, 3 female) with raised PCV (males greater than 0.51, females greater than 0.47) and normal red cell mass (apparent polycythaemia) and with normal red cell mass but a low plasma volume (relative polycythaemia) were studied retrospectively. Male sex, smoking, hypertension and diuretic therapy were found to be associated factors while obesity and excessive alcohol consumption were less clearly linked. Only three patients (all males) were negative for all these factors. Approximately 15% of patients had arterial hypoxaemia. There was no difference in the incidence of these factors in the two sub-groups. Possible mechanisms relating these factors to the raised PCV are discussed. Preliminary follow-up data show that in approximately one third of the patients the PCV returns to normal and that this is most likely in those patients with a normal recumbent PCV (at the time of the blood volume study). In one third the PCV continues to be raised, and the remainder have only intermittently raised PCV. As a result of this study, prospective studies of the incidence of hypoxaemia in these patients and of the PCV outcome with and without correction of associated factors are planned.