Background: Universal access to antiretroviral (ARV) therapy for HIV-1 infected patients has significantly increased in Tanzania since the last quarter of 2004. A total of 335,292 (76.2%) eligible clients are on ART. HAART regimens may vary in its ability to suppress viral load and to increase the CD4+ Lymphocyte cell count in people infected with different HIV-1 subtypes, possibly due to genetic diversity, virulence, and different pathways to suppress HIV replication. In order to better understand reasons for HAART response variability, HIV-1 infected patients who were on HAART for one year or more were recruited into the study. Method: A structured questionnaire was administered; demographic data, physical examination and blood collection were done. Review of previous clinical records was done to record past medical information including but not limited to immunological and virological data, side effects and ART regimen used before. Laboratory analysis was done during recruitment; CD4+ Lymphocytes, Complete blood count, HIV-1 Subtypes, Viral load and genotyping were the outcome. Results: The study population consisted of 129 HIV-1 infected patients with a median age of 45 years (IQR 20-72) of which 53 (41%) were males and 76 (59%) were females. There was 22% and 11% immunological and Virological failure using WHO criteria among study participants respectively. Females more often had immunological success than males. Of eighteen successful amplification, twelve (56%) were HIV-1 subtype C, 17% were subtype A and CRF01-AE each, while a unique combination of C and D, C and A accounted for 3% each. Seventy-five percent had resistance to non-nucleoside reverse transcriptase inhibitors (NNRTI), 19% had resistance to nucleoside reverse transcriptase inhibitors (NRTI) while only 1(6%) had resistance to protease inhibitors (PI). Twenty-two percent were detected to harbor multidrug resistance mutations such M184V, K103N T215C. Conclusion: The observed treatment failure is high as compared to previous studies done in Tanzania; all major HIV-1 subtypes are circulating in Tanzania. HIV-1 drug resistance mutation is high, especially in patients treated with NNRTI based combination.