The diagnosis of polycythaemia vera (PV) has been established upon sets of clinical criteria, which require the presence of absolute erythrocytosis (AE). The most recent clinical criteria for PV, published by the World Health Organization (WHO) in 2001, also required AE, and stated that the measured red cell mass (RCM) could be replaced by a surrogate marker for AE; a haemoglobin (Hb) value of >18.5 g/dl in males and >16.5 g/dl in females. The present study evaluated the potential of venous haematocrit (Hct) and Hb values as possible surrogate markers for AE in a series of 77 consecutive patients with PV and 66 patients with apparent polycythaemia (AP), in all of whom the RCM had been previously determined. In only 35% of the male PV patients would Hb values >18.5 g/dl indicate the presence of AE. Conversely, 14% of male AP patients would be misdiagnosed as having AE. A Hb > 16.5 g/dl would predict the presence of AE in 63% of the female PV patients, but 35% of female AP cases would be misdiagnosed as having AE. However, when the Hct was > or =0.60 an AE was always present, and this was true for both male and female subjects.