APOBEC3G and APOBEC3F restrict human immunodeficiency virus type 1 (HIV-1) replication in vitro through the induction of G-->A hypermutation; however, the relevance of this host antiviral strategy to clinical HIV-1 is currently not known. Here, we describe a population level analysis of HIV-1 hypermutation in [corrected] clade B proviral DNA sequences (n = 127). G-->A hypermutation conforming to expected APOBEC3G polynucleotide sequence preferences was inferred in 9.4% (n = 12) of the HIV-1 sequences, with a further 2.4% (n = 3) conforming to APOBEC3F, and was independently associated with reduced pretreatment viremia (reduction of 0.7 log(10) copies/ml; P = 0.001). Defective vif was strongly associated with HIV-1 hypermutation, with additional evidence for a contribution of vif amino acid polymorphism at residues important for APOBEC3G-vif interactions. A concurrent analysis of APOBEC3G polymorphism revealed this gene to be highly conserved at the amino acid level, although an intronic allele (6,892 C) was marginally associated with HIV-1 hypermutation. These data indicate that APOBEC3G-induced HIV-1 hypermutation represents a potent host antiviral factor in vivo and that the APOBEC3G-vif interaction may represent a valuable therapeutic target.