1. Children 2 years 0 months to 6 years 11 months of age will be recruited.
2. Delay in language and/or social development with or without delays in gross and/or fine motor development.
3. CSF BH4 level less than or equal to 30 nM/L as determined by lumbar puncture.
4. Adaptive Behavior greater than 50 as determined by the Vineland Adaptive Behavior Scale.

1. Epilepsy / Seizure disorder as determined by medical history.
2. Epileptiform discharges without clinical seizures.
3. Metabolic disorder that would be a contraindication to tetrahydrobiopterin treatment.
4. Genetic disorder
5. Therapies that cannot be maintained at a constant level or have not been ongoing for more than 3 months as determined by medical history
6. Prematurity
7. Developmental delay isolated to motor delay.
8. Current gastroesophageal reflux
9. Current or history of liver or kidney disease
10. Severe irritability (as determined by the aberrant behavior checklist)
11. Drugs known to affect folate metabolism (e.g., methotrexate) and their derivatives
12. Patients who are receiving drugs that affect nitric oxide-mediated vasorelaxation (e.g., PDE-5 inhibitors such as sildenafil, vardenafil, or tadalafil)
13. Patients who are receiving levodopa