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A decrease in the prevalence of drug resistance (DR) has been observed among recently infected (RI) individuals in Montreal. A study of chronically infected (CI) patients, who represent potential HIV-1 transmitters, was carried out in order to ascertain biological factors associated with this trend change.Retrospective analysis of CI patients was performed for the period 1996–2003. Changes in mean viral load and DR prevalence were assessed in CI patients (n = 2328) and compared to those in RI patients (n = 180) living in the same geographic area.A decrease was observed in the prevalence of DR among RI patients, from 13.0% in 1997–2000 to 4.0% in 2001–2003 (P = 0.04). From 1996 to 2000, the mean viral load in the CI patients decreased by 1.34 log10, to remain steady thereafter. The proportion of CI patients who interrupt treatment increased steadily over 1997–2003 from 3.1% to 16.5% (P < 0.0001). Since 1999, when genotyping analysis became available, we have observed a 0.9 log10 decrease in mean viral load among 602 genotyped CI patients harbouring any major mutations.The decrease in transmission of DR documented in Montreal since 2000 coincides with the drop in mean viral load observed in CI patients. Factors that contribute to the decrease in viral load include routine access to genotyping and availability of more potent antiretroviral drugs. Plasma viral load seems to represent the main predictor for the transmission of DR.