The primary rationale for this study is to define an optimal rAvPAL-PEG dose regimen by establishing the therapeutic effect within the shortest time possible time for induction, titration and maintenance phases while reducing the severity and frequency of hypersensitivity reactions that may lead to dose interruptions. It is hypothesized that these goals can be achieved by keeping rAvPAL-PEG doses low when anti-PEG IgM response is predicted to be high and titrating to an efficacious dose once the IgG response to PAL has developed. Further investigation is needed to determine how early and quickly patients can titrate safely to lower blood Phe; therefore, this protocol proposes to assess two Groups using an induction/titration and maintenance schedule with an aim towards establishing the therapeutic effect safety within an optimal period of time.