Seven new homozygous cases of hereditary triosephosphate isomerase (TPI) deficiency have been detected in five unrelated families. Two of the families originate in France, the others from Algeria, Yugoslavia, and Morocco. Only the parents coming from Algeria and Morocco were first cousins. In the other parents no evidence of consanguinity was found. All seven patients exhibited the same symptoms, i.e. hemolytic anemia appearing very early after birth associated with progressive neuromuscular symptoms. Expression of the deficiency is heterogeneous; this had previously been pointed out in the previously reported cases of TPI deficiency. Red cell TPI activity was 3 to 4% of the normal mean in the patients and 50 to 60% in the parents. The latter did not exhibit any clinical symptoms. The levels of red cell glycolytic intermediates and the characteristics of the mutated TPI could be studied in four of the patients only. Substantial increases of red cell dihydroxyacetone phosphate and of fructose 1,6-diphosphate, normal Km of TPI for glyceraldehyde phosphate, and thermoinstability of the enzyme were found. In addition the electrophoretic pattern showed no significant modification of the mobility of the TPI bands, but abnormal decreased staining of the two more anodal bands.