SummaryClinical, biochemical and ultrastructural assessment of five Black children from four unrelated kindreds, who had morphologic and laboratory features of hereditary pyropoikilocytosis (HP) is described. In two of the unrelated propositi, both with relatives having elliptocytosis, the apparent HP gradually evolved into elliptocytosis. The mode of inheritance appeared to be autosomal dominant in one family and autosomal recessive in the other. None of the three children from two separate families with true HP had a parent with HP or elliptocytosis. A sixth Black child, whose mother also had elliptocytosis, demonstrated typical elliptocytosis from birth and never demonstrated the HP phenotype. Sequential quantitation of heat induced red cell fragmentation using histogram analyses revealed unchanged findings in three patients with true HP but progressive amelioration in the two patients with HP phenotype who eventually developed elliptocytic morphology. Previously unreported lesions in both true and apparent HP patients included endovesicle formation and submembrane particles, which were detected by electron microscopy, an increase in Heinz body formation, and elevation of oxidized glutathione levels. Our study demonstrates shared abnormalities present both in some neonates with elliptocytosis and in neonates with HP, which preclude the diagnosis of HP in infancy and provide further evidence of the heterogeneity of the elliptocytosis syndrome.