Tetrahydrobiopterin (BH4) is a vital central nervous system (CNS) cofactor that maintains availability of amine neurotransmitters such as dopamine (DA) and serotonin (5HT), and stimulates and modulates the glutamatergic system. Dysregulation of these neurotransmitter systems has been implicated in the pathogenesis of schizophrenia (SZ). A central role of BH4 in schizophrenia is further supported by a study finding a relative deficit of 32% for SZ patients as compared to controls. The observed BH4 deficit is comparable to that reported for genetic BH4 deficiency disorders, supporting its characterization as having physiological significance. This highly significant finding, along with: a) the known roles of BH4 in neurotransmitter maintenance, b) dysregulation of CNS neurotransmitter synthesis observed in human BH4 deficiencies, c) evidence that plasma biopterin levels are correlated with CNS biopterin levels, and d) evidence that urinary biopterin excretion is not increased in SZ, all support our hypothesis that dysregulation of BH4 biosynthesis is involved in the pathophysiology of SZ. Moreover, additional data suggesting that mood stabilizer drugs can moderately increase levels of biopterin in patients with psychiatric disorders, and the reported efficacy of BH4 in pilot studies of neuropsychiatric disorders, suggest that alleviation of the schizophrenia BH4 deficit via treatment with synthetic BH4 supplement (Sapropterin dihydrochloride or Kuvan), may give rise to an improvement of the symptoms of schizophrenia, including positive and negative symptoms as well as neurocognitive deficits. Sapropterin dihydrochlorides, teh active pharmaceutical ingredient in Kuvan Tablets,is a synthetic preparation of the dihydrochloride salt of naturally occurring tetrahydrobiopterin (BH4)