Background: Resistance to antiretroviral therapy is a determining factor for therapeutic failure in HIV/AIDS. The prevalence of primary resistance (i.e. in those patients that have not received treatment) varies in different parts of the world. Aim: To study the prevalence of primary resistance to antiretroviral drugs in patients living in Northern Santiago. Patients and methods: Viral load, lymphocyte subpopulations by flow cytometry and genotypic resistance testing were assessed in blood samples from 60 HIV-1 infected patients (mean age 37 years, 54 male). Results: Mean CD4 cell count and viral load was 200 cells/ml and 142,840 RNA copies/ml respectively. Ten mutations were identified: V179D, L10I/V, M361, L63P, A71T/V, Y115F, V118I and K20R. None of these mutations is associated to a high degree of resistance to reverse transcriptase inhibitors, nucleoside analogs (NRTI), non nucleoside analogs (NNRTI) or viral protease inhibitors. Conclusions: This is a first approach to study antiretroviral resistance in Chilean patients. This study must be amplified, since the prevalence of resistance may experience changes with time (Rev Méd Chile 2005; 133: 295-301)