This is a competitive renewal application for The Philadelphia Pediatric AIDS Clinical Trials Unit (PACTU), a consortium consisting of The Children's Hospital of Philadelphia (CHOP) as the main unit and a site for pediatric studies, St. Christopher's Hospital for Children (SCHC) as a pediatric subunit, and the Hospital of the University of Pennsylvania (HUP) and Temple University Hospital (TUH) as perinatal subunits. The Philadelphia PACTU has achieved a strong performance record over the past four years, with enrollments into Pediatric AIDS Clinical Trials Group (PACTG) clinical trials increasing from 24 during the first year of operation to 63 during year 4. The Philadelphia PACTU has had excellent success in entering patients into primary therapy, opportunistic infection, perinatal transmission, adolescent, quality- of-life, and immune-based therapy protocols, including many Phase I studies. In the past two years, an aggressive effort to promote participation in perinatal studies has resulted in a dramatic increase in the number of subjects enrolled into perinatal protocols. The Philadelphia PACTU has an outstanding record of administrative performance and compliance with regulatory and data management procedures. Several members of the Philadelphia PACTU hold leadership positions in the PACTG and are making major contributions to the scientific agenda, particularly with regard to immune-based prevention and treatment of HIV-infection in children. It is anticipated that, over the next four years, the Philadelphia PACTU will continue to: 1) enroll subjects into high priority protocols; 2) enroll an increased number of subjects into pediatric and perinatal protocols, including Phase I studies; 3) maintain its excellent record of compliance with regulatory and data management procedures; and 4) contribute in a major way to the scientific agenda of the PACTG.