Published data from Europe and North America indicate that for non-iron-deficient adult Caucasian males, the normal mean packed cell volume (PCV) is 0.46 and the 2.5-97.5 percentile interval is 04.0-0.53. Corresponding values for adult Caucasian females are: mean PCV 0.42; 2.5-97.5 percentile interval 0.36-0.48. It is not usually appropriate to undertake studies of polycythemia in adult Caucasian males with PCV < 0.55 (Hb Conc. < 180 g/L) or in adult Caucasian females with PCV < 0.50 (Hb Conc. < 16.5 g/L). Application of this principle will reduce the number of inappropriate and costly studies that would otherwise be performed in patients whose PCV values are only in the upper percentiles of the normal range, and will help to avoid misdiagnoses and therapeutic misadventures.