Histidinemia is a metabolic disorder characterized by increased levels of histidine in blood, urine, and cerebrospinal fluid, and decreased levels of the metabolite urocanic acid in blood, urine, and skin cells. Although histidinemia was originally associated with mental retardation and speech defects, it is generally considered to be a benign disorder (Levy et al., 2001). However, it is possible that histidinemia may be a risk factor for developmental disorders in certain individuals under specific circumstances, such as perinatal events (Ishikawa, 1987).