Platelet counts and platelet size were determined in midtrimester fetuses to obtain reference values to be used in prenatal diagnosis of inheritable platelet disorders. In 17- to 21-week fetuses, platelet counts ranged between 135 and 283 x 10(9)/L, and thus did not differ from those of newborn infants and adults. Mean platelet volume fell between 6.0 and 7.0 fl, which is in the range for adults. The data were used to exclude Wiskott-Aldrich syndrome in an 18-week fetus at 50% risk of being affected.