Background: Current guidelines for HIV management recommend monitoring plasma HIV-1 RNA level every 3-6 months in patients on a stable antiretroviral regimen. However, cost is the major obstacle to follow the guidelines in resource-limited settings. Objective: This study aimed to compare the outcome of antiretroviral therapy among HIV-infected patients on a stable regimen who had plasma HIV-1 RNA monitoring once vs twice yearly. Methods: A retrospective cohort study was conducted among HIV-infected patients receiving antiretroviral therapy since 2002 at Chiang Mai University Hospital, Thailand. We evaluated the incidence of virological failure and number of reverse transcriptase (RT) mutations between groups. Results: Of 551 patients on a stable antiretroviral regimen, 405 (73.5%) and 146 (26.5%) patients had plasma HIV-1 RNA measurement once and twice yearly, respectively. Forty-seven of 405 patients (11.6%) in once-yearly group and 15 of 146 patients (10.3%) in twice-yearly group developed virological failure, giving the incidence rate of 2.03/100 and 1.95/100 person-years, respectively. The probability of virological failure did not differ between groups (p=0.897, log-rank test). The number of RT mutations was not statistically different between groups (all p-values>0.05). The predicting factors for virological failure from a multivariate analysis were adherence rate <95% and baseline CD4 cell count <50 cells/mm3 but not the frequency of HIV-1 RNA monitoring. Conclusions: The incidence of virological failure and the number of RT mutations were not different between groups. Therefore, in resource-limited settings, the recommendation to perform plasma HIV-1 RNA measurement once yearly in patients on a stable antiretroviral regimen is justified. © 2011 Bentham Science Publishers Ltd.