Direct interactions at the cellular level in vitro have been reported which suggest that opportunistic viruses may reactivate latent human immunodeficiency virus (HIV) in cells. The significance of these findings depends on whether coinfection of the same cell with these two different types of viruses occurs in vivo. Using various double-labeling techniques, we present evidence that human cytomegalovirus (HCMV) and HIV can coinfect the same cell in vivo in central nervous system tissue from AIDS patients. These observations indicate that direct cooperation at the single cell level could occur between HCMV and HIV. This new finding in the context of reports that herpesviruses can increase HIV transcription in vitro, suggest the possibility of a direct role for herpesviruses in the pathogenesis of AIDS.