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To assess temporal changes in prevalence of transmitted HIV-1 drug resistance in a homogeneous cohort of newly infected individuals.Pretreatment genotypic and phenotypic drug resistance was tested in 154 subjects with primary HIV-1 infection identified between 1995 and 2001 (group A; n = 76) and 1999 and 2001 (group B; n = 78). Sequence analysis was assessed by population-based sequencing. Virus susceptibility to antiretroviral agents was determined by the PhenoSense assay (ViroLogic).The frequency of resistance-associated mutations in protease (PR) and reverse transcriptase (RT) genes increased from 13.2% (1995–1998) to 19.7% (1999–2001). Although the overall prevalence of viruses with phenotypic resistance did not vary (1995–1998, 10.0%; 1999–2001, 10.8%), the distribution of drug classes changed [nucleoside RT inhibitor (NRTI): 8.3% to 2.7%; non-NRTI: 5.0% to 8.1%; protease inhibitors (PI): 1.7% to 5.4%]. The decrease of phenotypic resistance to NRTI in 1999–2001 was caused by the absence of transmitted lamivudine-resistant variants. Phenotypically susceptible variants with aspartic acid or serine residues at position 215 of RT (5.3%;P = 0.04) instead emerged. Hypersusceptibility to PI decreased from 18.3% to 5.4% (P = 0.02) while the amino acid substitutions in PR increased over time: M36I (6.6% to 19.7%) and A71V/T (3.9% to 15.8%).There was an increase in the number of HIV-1 variants with both genotypic and phenotypic resistance to non-NRTI and PI over time. Furthermore, viruses with altered genotypes compatible with thymidine analogue or PI exposure but susceptible phenotypes were seen in 1999–2001. The latter findings suggest transmission of viruses from subjects who have either changed or discontinued therapy.