In the diagnosis of polycytemia vera, estimation of erythrocyte volume (EV) from plasma volume (PV) and venous hematocrit (Hctv) is usually thought unadvisable, because the ratio of whole body hematocrit to venous hematocrit (f ratio) is higher in patients with splenomegaly than in normal subjects, and varies considerably between individuals. We determined the mean f ratio in 232 consecutive patients suspected of polycytemia vera (f = 0.967; SD 0.048) and used it with each patient's PV and Hctv to calculate an estimated normalised EVn. With measured EV as a reference value, EVn was investigated as a diagnostic test. By means of two cut off levels the EVn values could be divided into EVn elevated, EVn not elevated (both with high predictive values), and an EVn borderline group. The size of the borderline EVn group ranged from 5% to 46% depending on position of the cut off levels, i.e. with the efficiency demanded from the diagnostic test. EV can safely and rapidly be estimated from PV and Hctv, if f is determined from the relevant population, and if the results in an easily defineable borderline range of EVn values are supplemented by direct EV determination.