The HIV-1 trans-activator (Tat) protein is proposed as an important factor in the complex HIV-induced pathogenesis of AIDS. In this paper, multiple effects of this viral protein are described. Originally discovered as an intracellular activator of HIV-1 transcription, Tat was found to regulate viral reverse transcription as well. Trans-activator was found to be secreted by HIV-infected cells and taken up by neighbouring cells. In this way, Tat is able to affect both infected and uninfected cells. Intracellularly, Tat can deregulate the expression of several heterologous cellular and viral genes. Extracellular Tat can contribute to the spreading of HIV-1 and immunosuppression of uninfected cells. Finally, there is evidence that exogenous Tat is involved in AIDS-associated pathologies such as Kaposi's sarcoma and HIV-associated dementia. These capacities together accelerate the progression towards AIDS and make Tat an interesting candidate as a constituent of an anti-AIDS vaccine.