Human Immunodeficiency Virus (HIV) in addition to its profound effect on the immune system has been shown to produce a primary and persistent infection of the Central Nervous System (CNC) with relentlessly progressive encephalopathy. The sequelae of this infection are particularly devastating in the pediatric age group. HIV infection in infants and children leads to microcephaly and brain atrophy. Opportunistic CNS infections in pediatric AIDS are rare. The mechanisms of cellular damage and the cell population in the CNS which is primarily damaged by HIV are still not known. The objective of this study is to define the effect of HIV infection on a developed and on a developing brain; to define the extent of CNS atrophy/microcephaly; and to determine which brain cell populations are affected in pediatric AIDS. Brains obtained at autopsy from infants and children with AIDS infected with HIV either in utero, or postnatally by blood transfusion or with blood products, and the brains from non-AIDS age-matched control cases will be evaluated neuropathologically. Histological, stereological, morphometric and immunocytochemical methods will be used. The information from this study will contribute to a better understanding of the effect of HIV infection of the developing human brain and for better clinico- pathological definition of HIV encephalopathy in pediatric AIDS.