The Pediatric Branch of The Clinical Oncology Program (COP) has become a national leader in pediatric AIDS research. Since dementia, encephalopathy, and neuropsychological deficits are well recognized manifestations of HIV infection in both adults and children, longitudinal serial assessments are necessary. The incidence of these deficits and its further progression is not well understood in either population. Therefore, longitudinal prospective neurocognitive and behavioral assessments allow COP to monitor the course of the disease and benefits of therapeutic interventions. Comprehensive age appropriate assessments will be performed on all patients with HIV infection, particularly those who may be eligible for trials evaluating anti-retroviral therapy. Repeat assessments will be administered at variable (3 - 12 month) intervals, to monitor the impact of the therapy on psychological functioning. The careful assessment of therapy related change is an essential aspect of the evaluation of new therapeutic modalities. In addition, techniques will be developed to more comprehensively assess these patients and treatment related changes.