For several common hematological quantities, the within-subject biological variation during a 1-year period was estimated in a group of 39 apparently healthy volunteers: 19 women and 20 men. All measurements were performed with a hematological analyzer (Toa E-5000, Toa Medical Electronics, Kobe, Japan). The means of the within-subject biological coefficients of variation, separated by sex when significant difference was shown, were as follows: 2.8% for hemoglobin concentration; 3.0% for erythrocyte concentration; 2.9% for erythrocyte volume fraction; 1.1% for mean erythrocyte volume; 1.9% for arbitrary SD of erythrocyte volume; 1.6% for mean erythrocyte hemoglobin concentration; 1.3% for mean erythrocyte hemoglobin; 10.8% in men and 15.7% in women for leukocyte concentration; and 6.7% in men and 8.6% in women for platelet concentration. The desirable between-day imprecisions for the measurement of each quantity were established as one half of the average of within-subject biological variation.