HIV-1BR was isolated from the autopsied brain tissue of a 57-year-old man who died of progressive dementing illness. This virus was shown to be HIV-1 by hybridization to HIV-specific DNA probes. The expression of viral proteins as tested by radioimmunoprecipitation assay revealed the presence of HIV-1-specific proteins. HIV-1BR replicated in cultures of CD4+ T-lymphoid cells and induced cytopathic effects in these cells. HIV-1BR also replicated in monocytoid cell lines. The genetic nature of this isolate was determined by molecular cloning and sequencing of the 3'-half of the genome. DNA sequence information established that HIV-1BR is a unique HIV-1 isolate. A stretch of approximately 30 bases in the nef gene of HIV-1BR was found duplicated when compared with the other sequenced HIV-1 genomes. The functional significance of this duplication remains to be determined.