A clear and consistent genetic classification of human immunodeficiency virus-type 1 (HIV-1) strains continues to be of great utility in epidemiological tracking of the AIDS pandemic and in vaccine design. It also provides a foundation for detecting any biological differences that may have evolved during the diversification of these viruses. Over the past few years, expanding access to diverse HIV-1 samples from throughout the world, coupled with improved polymerase chain reaction (PCR) amplification and sequencing technologies, has led to the discovery of ever-increasing numbers of sequences that do not fit well into the standard HIV-1 subtyping nomenclature. Two issues of particular importance that have needed to be resolved were the classification of related recombinant viruses that are epidemic strains and the minimum criteria for designating a new subtype. A working group was convened to discuss HIV-1 nomenclature, and the participants arrived at a proposal (1) that resolves current ambiguities and retains as much as possible of the nomenclature system that has been adopted over the past 10 years. The salient points of the proposal are the following.