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The Research Capacity ProgramAs in many areas of Brazil, the AIDS epidemic in Curitiba is relatively stable, but surveillance is important to support public policy. The molecular characteristics of HIV may be instrumental for monitoring epidemic trends. We evaluated plasma HIV-1 RNA (n = 37) from 38 cases presenting with positive serology, who were among 820 consenting volunteers visiting the downtown counselling and serology testing centre. Seroprevalence was 4.6% 3) and the estimated HIV incidence, as defined by the BED assay, was 2.86 persons/years ). An additional set of contemporaneous, anonymous samples from a local laboratory was also analysed (n = 20) . Regions of the HIV-1 polymerase (n = 57) and envelope (n = 34) were evaluated for subtyping, determination of mosaic structure, primary drug resistance mutations (pDRM), envelope V3 loop motifs and amino acid signatures related to viral tropism. HIV-1 clade B was observed in 53% of cases; HIV-1C in 30% and BC mosaics in 14%, with one F genome and one CF mosaic. Clade C infection was associated with recent infections among males (p < 0.03). Stanford surveillance pDRM was observed in 8.8% of sequences, with 7% showing high level resistance to at least one antiretroviral drug. Tropism for CXCR4 co-receptor was predicted in 18% of envelope sequences, which were exclusively among clade B genomes and cases with serological reactivity to chronic infection.Key words: epidemiology -antiretroviral resistance -genetic diversity -HIV-1 -tropism -BrazilUnderstanding the local HIV-1 epidemic will not only support the regional response, but it may also provide information to nationwide efforts in the fight against AIDS. The variability of HIV-1 has been an obstacle to treatment and to the development of prevention innovations such as vaccines. HIV-1 clusters phylogenetically into distinct clades (subtypes), a fact that has helped to unravel the molecular characteristics of the epidemic. In some regions, differential distribution of HIV-1 clades has been documented (Van Harmelen et al. 1997, Tovanabutra 2001, Rios et al. 2005, Ryan et al. 2007) and phylogeny may indicate the source(s) of incoming variants (Sarker 2008). Worldwide, clade C is responsible for about half of the epidemic, whereas clade B is the major variant in industrialised nations (Hemelaar et al. 2006). HIV variability may influence the performance of laboratory tools for diagnostic, monitoring and surveillance (Koch et al. 2001 (Baeten et al. 2007, Kaleebu 2007 and transmission potential (Yang et al. 2003, John-Stewart et al. 2005 have been associated to distinct HIV clades, but the actual impact of genetic diversity in HIV disease still remains uncertain (Stebbing & Moyle 2003, Hemelaar et al. 2006, Tebit et al. 2007). However, some characteristics of HIV, such as tropism to either CCR5 or CXCR4 coreceptors, seem to be associated with a distinct pattern of disease progression (Shepherd et al. 2008). Another relevant issue is the identification of primary drug resistance mutations (pDRM) among untreate...