Full text article. Also available at https://doi.org/10.11604/pamj.2016.23.26.8268Moderate to severe anemia is an important clinical problem in HIV patients on Highly Active Antiretroviral Therapy. The rate of progression and mortality in this sub group of patients is high compared to non anemic patients. In sub Saharan Africa with scale up of Anti retroviral therapy, the magnitude of this problem is not known especially in Tanzania. This study aimed at determining the magnitude and correlates of moderate to severe anemia in HIV patients receiving first line ART in northwestern Tanzania. This was a cross sectional clinic based study, involving adult HIV patients on first line Highly Active Antiretroviral Therapy at Bugando Medical Centre Care and Treatment Center. The patients' data were analyzed using STATA version 11 to determine the prevalence of moderate to severe anemia and risk factors that could predict occurrence of anemia. Results: In this study 346 patients on Highly Active Anti-Retroviral Therapy were enrolled, of whom 100(40.46%) had moderate to severe anemia. The odds of being anemic were strongly predicted by Zidovudine based regime, low baseline CD4 count (< 200 cells/µl) and HIV stage 3&4 at enrollment. Most of the anemic patients had mean corpuscular volume of >100fl. The prevalence of moderate to severe anemia is significantly high in this cohort of HIV-infected patients on first line Anti Retroviral Therapy and it is strongly predicted by Zidovudine based regime, low baseline CD4 and HIV stage 3 and 4. On clinical grounds this suggests that patients who are initiated on Zidovudine based regimen and those in advanced HIV at enrollment should have regular haemoglobin follow up to identify anemia at its earliest stage to improve the clinical outcome of these patients.