In this open-label, multicenter, multinational study, ZDV is replaced with d4T in ZDV-containing regimens, or d4T is added to non-ZDV-containing regimens in 20 patients experiencing ADC. Patients are defined as having failed treatment if they progress by one ADC stage on the MSK (Memorial Sloan Kettering) rating scale on study (i.e., from Stage 1 to 2 or Stage 2 to 3). Patients are evaluated on a weekly basis until the dementia deterioration is confirmed to be caused by HIV-1. The effect of d4T-containing regimens is assessed for the following parameters: neurological status, survival, AIDS-defining conditions, CSF (cerebrospinal fluid) and plasma viral load, CSF and blood immunological markers, blood CD4 cell counts, and viral resistance. This study also assesses the pharmacokinetics of d4T in the CSF and in the blood.