The AIDS dementia complex (ADC) is the SNC complaint that appears most frequently in AIDS patients. ADC is characterized by a subacute onset of dementia accompanied by motor disturbance and changes in behaviour and is considered to be directly caused by HIV-1. Very frequent in advanced stages of AIDS, it can also be the way in which the illness appears. In 90% of the necropsies of these patients a diffuse demyelination of the white substance is observed with multinucleate cells appearing in 40% of the cases. Up to now, the factors triggering this disease are not well understood, nevertheless, the immunosuppression present in these individuals could act as a factor favouring the appearance of ADC. A specific marker does not exist and, therefore, the complementary studies can only help to eliminate other causes of neurological complaints. Successful tests of treatment with zidovudine have been made, although it would be necessary to carry out studies with a larger number of patients to be able to evaluate its long-term efficacity.