Possible causes for a normocytic hyperregeneratory anemia are beside an incomplete treatment of iron deficiency, vitamin B12 deficiency or folic acid deficiency notably a hemolysis. After exclusion of other causes of hemolysis like immune hemolytic anemias, microangiopathic hemolytic anemias and hemoglobinopathies, an enzyme deficiency of erythrocytes should be considered. By far the most common form worldwide is the Glucose-6-phosphate deficiency. In the most frequent variants of this disease hemolysis occurs only during stress, imposed for example by infection, "oxidative" drugs or after ingestion of fava beans. The most serious clinical complication of the Glucose-6-phosphate deficiency is the rarely observed neonatal icterus. Some enzyme variants can cause chronic hemolysis which is described as chronic nonsperocytic hemolytic anemia. This form of chronic anemia can also be caused by other enzyme deficiencies, most frequently by the Pyruvate kinase deficiency. All other deficiencies of glycolytic enzymes are even rarer. It should be noted that in some of these very rare forms neurological rather than hematological symptoms predominate the clinical syndrome. If there is suspicion, on the basis of clinical symptoms and/or familial history, diagnosis of an enzyme deficiency can be achieved relatively easy by measurement of the enzyme activity. Accurate diagnosis might be helpful in therapeutic decisions (e.g. splenectomy in certain forms) and it is essential for genetic counseling, since certain deficiencies are transmitted as autosomal recessive disorders (e.g. pyruvate kinase deficiency), while the most common form, the glucose-6-phosphate dehydrogenase deficiency is linked to the X-chromosome.