Heinz bodies (HBs) are uncommon except with G6PD deficiency immediately following hemolysis, postsplenectomy, and in patients with unstable hemoglobin variants. They are present characteristically in the congenital Heinz body hemolytic anemias (CHBHA - the unstable hemoglobinopathies). There are now some 200 different identified molecular variants of hemoglobin underlying CHBHA. Less than one-half of these are of clinical significance. The three major causes for HB formation and increased hemolysis are exposure to certain chemicals and drugs, deficiency of one of the reducing systems of blood, and presence of an unstable hemoglobin. Oxidative denaturation of the hemoglobin molecule leads to HB formation with the first two situations and is probably the mechanism for the precipitation of unstable hemoglobin.