Rationale: Since 2001, antiretroviral therapy (ART) for people living with HIV (PLHIV) has been available in the Lao People's Democratic Republic (PDR). A key factor in the effectiveness of ART is good adherence to the prescribed regimen for both individual well-being and public health. Poor adherence can contribute to the emergence of drug resistant strains of the virus and transmission during risky behaviors. Increased access to ART in low-income country settings has contributed to an interest in treatment adherence in resource-poor contexts. This study aims to investigate the proportion of adherence to ART and identify possible factors related to non-adherence to ART among people living with HIV (PLHIV) in Lao PDR. Methods: A cross-sectional study was conducted with adults living with HIV receiving free ART at Setthathirath hospital in the capital Vientiane and Savannakhet provincial hospitals from June to November 2011. Three hundred and forty six PLHIV were interviewed using an anonymous questionnaire. The estimation of the adherence rate was based on the information provided by the PLHIV about the intake of medicine during the previous three days. The statistical software Epidata 3.1 and Stata 10.1 were used for data analysis. Frequencies and distribution of each variable were calculated by conventional statistical methods. The chi square test, Mann-Whitney test and logistic regression were used for bivariate analyses. Multiple logistic regression analysis was conducted to determine the predictors of non-adherence to ART. A p-value < 0.05 was considered to indicate statistical significance. Results: Of a total of 346 patients, 60% reported more than 95% adherence to ART. Reasons for not taking medicine as required were being busy (97.0%), and being forgetful (62.2%). In the multivariate analysis, educational level at secondary school (OR=3.7, 95% CI: 1.3-10.1, p=0.012); illicit drug use (OR=16.1, 95% CI: 1.9-128.3, p=0.011); dislike exercise (OR=0.6, 95% CI: 0.4-0.9, p=0.028), and forgetting to take ARV medicine during the last month (OR=2.3, 95% CI: 1.4-3.7, p=0.001) were independently associated with non-adherence. Conclusions: Non-adherence to ART was associated with individual factors and exposure to ART. Priority measures to increase adherence to ART should aim to intensify counseling and comprehensive interventions, such as guidance for PLHIV on medication self-management skills, tailoring the regimen to the PLHIV life style, and improving adherence monitoring and health care services.