Two brothers, 6 and 13 years old, had histidinemia. On the basis of clinical and biochemical observations, the younger boy was considered to have a classical type of the disease, while the older boy had an atypical form characterized by partial impairment of the skin histidase activity and a moderately prolonged half-life of blood histidine. The mother is a heterozygous carrier, while the father and sister seem to be normal.