According to available statistics of 2000, an estimated 2.2 million people in Kenya were reported to be living with HIV/AIDS while in 2007, 1.4 million people in Kenya were estimated to be living with HIV/AIDS (WHO, 2007). Total number of people on ART in Kenya in 2011 was estimated at 470,000 (NASCOP, 2011). Before people are initiated on ART, basic knowledge on HIV/AIDS is needed, understanding of opportunistic infections, ART and the side effects of treatment. To determine patients knowledge on various aspects of antiretroviral therapy. This was a descriptive cross-sectional study. A questionnaire was administered to 342 patients receiving antiretroviral therapy at Industrial Area Remand Prison and Loco health centers. The ratio of males to females of all participants was 1.4:1. Statistical significance was found between participants level of education and their knowledge on, definition of AIDS (p=0.006), definition of side effects (p=0.005), definition of ARVs (p=0.000), daily dosing time of ARVs (p=0.000), consequences of missing ARV doses (p=0.001) and duration of use of ARVs (p=0.004).