Dissertation (MMED Internal Medicine)Virological failure is one of the common challenges facing HIV-infected adults on antiretroviral therapy programmes particularly in resource limited settings. Objective: This study was done to determine the prevalence of virological failure and associated factors among HIV-infected adults receiving second line antiretroviral therapy in Dodoma city, Tanzania. Methodology: A cross-sectional analytical hospital-based study of HIV-infected adults on second line antiretroviral therapy was conducted at care and treatment centres of, Dodoma Regional Referral Hospital and Makole Health Centre. Data collected included social demographic characteristics, antiretroviral drug history, body mass index, Full Blood Picture, CD4 cell counts and viral load. Data were analysed using SPSS version 23. Correlation between viral load and CD4 cell counts were done by using Pearson correlation test. Association between categorical variables and continuous variables were done by chi-square test and t-test respectively. Logistic regression analysis was done to determine predictors of virological failure. A two-tailed P<0.05 was considered statistically significant. Results: A total of 249 HIV-infected adults on second line antiretroviral therapy were included in this study. The virological failure was 47 (18.9%) and the vast majority35 (74%) were on Atazanavir/ritonavir-based protease inhibitor. The predictors of virological failure were low CD4 cell count<350cells/μl(AOR=41.85, 95%CI=3.7-472.8, P=0.003) and low haemoglobin level(<12g/dl)(AOR = 0.251, 95% CI= O.160-O.393, P=0.001). Conclusion: In this study, 18.9% of HIV-Infected adults on second line ART had high viral load and could not suppress the viral load three months later. Low haemoglobin level and CD4 cell count<350cells/mL were the factors associated with virological failure.