This is an open-label extension study available only to subjects who completed an earlier double-blind, placebo-controlled study of sapropterin in children with autism. During this protocol, all subjects will be receiving brand-name Kuvan 20 mg/kg/day for 16 weeks; subject who complete the first 16 weeks will have the option of continuing on Kuvan at the same dose for up to 90 days after the last subject has completed the first 16 weeks of this protocol. The purpose of the study primarily is to gather additional information on safety and efficacy in this population.