The quantitation of platelets in peripheral blood is a well-recognized tool. However, other platelet parameters that recently have become available on a routine basis with the introduction of automated cell sizers may become increasingly important in evaluating the integrity of the thrombocytic function. These parameters include mean platelet volume (MPV) and platelet-crit (PCT). This study established reliable reference ranges for these platelet parameters while taking into consideration the effects of age and sex in a study population of 447 normal persons. There was an inverse, nonlinear relationship between MPV and platelet count, with no statistical difference (P greater than 0.05) seen between males and females. Platelet-crit showed no variation with respect to age or sex, however, a direct, linear relationship was suggested between PCT and platelet count. These relationships provide a better understanding of these platelet parameters and may contribute to their use as helpful diagnostic aids.