Objective: To characterize the specificities of HIV-1–related encephalopathy in children. Methods: Comparison of patients from the French Perinatal Cohort of children born to HIV-1–infected mothers and followed from birth with the French SEROCO Cohort of adults with a known date of infection. Our study examines 1) the characteristics of encephalopathy with onset before 1 year, after 1 year, and in adults, and 2) the maternal and birth characteristics of infants who developed AIDS before 1 year and went on to develop either encephalopathy or opportunistic infection. Results: The incidence of encephalopathy was higher in children than in adults during the first year (9.9% versus 0.3%) and intermediate during the second year (4.2% versus 0%) after infection but was similar thereafter (less than 1% per year in each group). The resulting cumulative incidence at 7 years postinfection reached 16% in children and 5% in adults. Encephalopathy that developed before 1 year 1) was more frequently an isolated symptom of AIDS, 2) was associated with a reduction of intrauterine brain growth, 3) was associated with a very low level of HIV-1 RNA in CSF, 4) occurred at a higher level of immunocompetence after taking into account the decrease in CD4 lymphocytes with age, and 5) was not prevented by zidovudine treatment during gestation. Conclusions: Early encephalopathy in infants has a different pathophysiologic mechanism than that occurring in children, which in turn shows similarities with that observed in adults. Early encephalopathy is probably related to the occurrence of pathologic events during late fetal life.