 Understanding whether or not viral replication occurs in the brain during chronic untreated HIV-1 infection is of undeniable importance, and has implications for treatment and research priorities. Evidence suggests that viral replication in the CNS occurs at the extremes of HIV-1 disease. Brain involvement has been reported during acute infection, and there is convincing evidence of CNS viral replication during HIV-associated dementia HAD and advanced AIDS. Some human and primate data suggest that viral RNA and proteins may be absent from brains of some individuals with chronic untreated HIV-1 infection despite abundant proviral DNA. However, the extent of viral replication in the brain is not known for most of the 42 million people worldwide living with untreated HIV-1 infection. 