To determine the manner in which erythrocyte changes occur during ontogeny, several red cell parameters were analyzed in fetuses, newborn infants, children, and adults. Although mean cell volume (MCV) and hemoglobin F (HbF) levels decreased as expected during in utero development, the coefficient of variation of red cell size (%CV), or red cell distribution width (RDW), increased from fetuses to newborn infants. In normal adults, the %CV was 15 (RDW was 13). The %CV in fetuses at 18-24 weeks gestation was 18, and it was 21 at term birth. High values for %CV or RDW indicate significant anisocytosis. Erythropoiesis at the time of birth is not a steady-state condition. Erythrocytes of a wide variety of sizes are present, with the appearance of new small cells on a background of older, larger red cells. This increased anisocytosis suggests that these new erythrocytes do not appear to be due to a smooth, continuous evolution of red cell size, but rather to discrete, perhaps clonal, changes.