This prospective study evaluated the electroencephalographic (EEG) diagnostic and prognostic value in childhood HIV infection. It was carried out on 125 subjects and included all Piemonte's seropositive children. The EEG was repeated every three months during the first 15 months of life, and then, at least, annually in the P1 and P2 group. Data of group P2 was compared blindly to that of the seroconverted control group of the same age and risk. EEG results were normal in P0, P1 and control patients. In group P2, EEG was abnormal in 35.5% of subjects, of these 54.6% developed an encephalopathy with a delay of 2.5 months to 2 years 11 months. EEG is therefore useful to evaluate early CNS damage and to identify onset features and evolution of encephalopathy in P2 patients.