Triose phosphate isomerase (TPI) deficiency is associated with a syndrome of congenital non-spherocytic haemolytic anaemia, mental subnormality, motor impairment, growth failure and cardiac failure. The deficiency state is characterized by moderately reduced red cell TPI activity, and marked instability of the abnormal enzyme to heat. The clinical features of an affected child are described, and some problems in the laboratory diagnosis delineated.