Objective, To determine the effects of combination antiretroviral therapy including a protease inhibitor (PI combination therapy) in children with advanced HIV-1 disease, Study design, An observational study of HIV-1 plasma RNA, lymphocyte subsets, delayed type hypersensitivity and physical growth after initiation of PI combination therapy. Results, In nine children the median HIV-1 plasma RNA decreased 1.7 log(10) (mean, 1.57; range, 0.7 to 2.2) following PI combination therapy and CD4 cells increased a median of 499 (mean, 528; range, 9 to 1088) cells/mu l. A rebound of RNA, associated with the development of resistance to the PI, occurred in three subjects, Three of six children were no longer anergic and all nine achieved normal weight-growth velocities, Ritonavir was well-tolerated, despite its bitter taste; however, four of five children treated with indinavir developed renal complications, Conclusions, PI combination therapy in children with advanced HIV-1 disease was associated with a decrease in HIV-1 RNA, improved immunologic measures and normal or better weight gain, Of concern was the rebound in plasma HIV-1 associated with resistance to the PI observed in one-third of patients, This emphasizes the need for larger studies to define optimal PI containing regimens with long term efficacy in children.