Two sisters with a rare inborn error of histidine metabolism resulting from urocanase deficiency are being presented. The more common form of familial histidinemia due to histidase deficiency is excluded. The urocanase deficiency is proven by demonstrating increased excretion of metabolites of the product of the urocanase enzyme action. Further, the strongest evidence for the urocanase defect rests on the demonstration of urocanase deficiency and normal histidase activity in liver.