Previous studies in adults have demonstrated that the mean platelet volume (MPV), which is now routinely measured during blood counting performed by the Coulter Counter S Plus, is an important parameter and may serve as a predictor of the hemostatic potential of patients with severe thrombocytopenia. MPVs and platelet counts were determined in 155 neonates in order to establish the normal values of the mean platelet volume during the neonatal period. Analysis of variance performed in 119 neonates of birthweight greater than 2 kg revealed a significant rise with age of both platelet count (P less than 0.0001 and MPV (P less than 0.02) during the neonatal period. The platelet counts of these 119 neonates (323336 +/- 106918; mean +/- SD) were significantly higher (P less than 0.05) than the platelet counts (283805 +/- 114130) of 36 neonates where birthweight was less than 2 kg. There was no significant difference between the MPV values of the two groups (8.67 +/- 1.03 micron3 and 8.89 +/- 1.10 micron3, respectively). Since increased MPV is observed during periods of platelet production, the above results suggest that the rise of platelet count associated with increased MPV in newborn infants is due to increased production of platelets in the early postnatal weeks.