• Trough concentration of LPV and pharmacokinetic parameters [ Time Frame: Weeks 8, 16, 24, 36, 48, 60, 72, 84, and 96 ]
  
• Recurrent treatment-related Grade 3 and non-life threatening Grade 4 toxicity or single occurence of life-threatening Grade 4 toxicity [ Time Frame: Throughout study ]
  

• Change in CD4 and CD8 count and percentage from baseline. HIV-1 specific CD4 count and CD8 mediated and humoral responses [ Time Frame: Study entry and Weeks 12, 24, 36, 48, 60, 72, 84, and 96 ]
  
• Time to virologic failure [ Time Frame: Throughout study ]
  
• Suppression of viral load to less than 400 copies/ml and less than 50 copies/ml [ Time Frame: Study entry and Weeks 2, 4, 8, 12, 16, 20, 24, 32, 36, 40, 48, 60, 72, 84, and 96 ]
  

LPV/RTV has shown significant antiviral activity and tolerability in clinical trials in adults and children over 6 months of age. LPV/RTV has been approved by the FDA to treat older children and adults with HIV. Data for children under 6 months, however, have not been available. LPV/RTV has not been approved for infants less than 6 months old, and the appropriate dose for young infants is not known. Dosing guidelines are needed for young infants, most of whom are in the early stages of primary infection. This study will help identify an appropriate dose range of LPV/RTV and evaluate response to therapy in infants less than 6 months of age. The study will also evaluate whether early therapy allows normal development of the immune system.

Infants between 14 days and 6 months of age will receive LPV/RTV in combination with 2 nucleoside reverse transcriptase inhibitors (NRTIs) chosen by their physicians. Twelve-hour pharmacokinetic sampling is performed on Day 14 of drug treatment and when the patient reaches 12 months of age. Patients will undergo a physical exam, medical history assessment, and blood collection at selected study visits. Study visits will occur every 2 weeks for the first 8 weeks, then every 4 weeks until the end of the first year of the study. Study visits during the second year will be every 12 weeks until the end of the study. The parent or guardian will be contacted by phone every 6 weeks to monitor adverse drug effects. Participants between the ages of 6 weeks but less than 6 months old will be followed for 96 weeks after the enrollment of the last participant. Participants between the ages of 14 weeks but less than 6 weeks will be followed for 48 weeks after the enrollment of the last participant.