Forty-three AIDS scientists from Europe, Africa, the United States, Canada, India and China met in Dar es Salaam, Tanzania to discuss the implications of the global variation of HIV (list of participants included in Appendix). This meeting followed an earlier meeting of the Joint United Nations Programme on HIV/AIDS, held in 1996 in Berlin, Germany , in which HIV genetic variability was considered in relation to transmissibility. During the Tanzania meeting, available data pertaining to the biological consequences of HIV genetic variation and its ramifications with regard to epidemiology, diagnostics, classification, and vaccine design were systematically reviewed. There was consensus that classification based on genetically defined subtypes provides an important framework for making advances on understanding viral biology and immunology, and for vaccine development. In addition, other groupings of viruses based on immunological and biological characteristics would be also valuable and help to further refine our understanding of the implications of variability. Key elements of the discussion are summarized here in the context of a review of the current literature.