The national programme of expanded access to antiretroviral therapy in the South African public health sector has resulted in hundreds of thousands of South Africans being subjected to prolonged therapy with the risk of adverse drug effects. Among the most common adverse effects are metabolic disorders one of which is mitochondrial toxicity. Mitochondrial toxicity may manifest as hyperlactatemia. The study was designed to determine the frequency with which hyperlactatemia occurs in HIV – infected adults on long-term antiretroviral therapy (ART). The objective was to determine the proportion of patients with blood lactate levels that exceed a predetermined cut-off level and to attempt to relate hyperlactatemia to a set of factors namely, gender, age, obesity, symptoms, type of ART regime and duration of ART use. The study was conducted at an ART clinic in the provincial state hospital of Bongani in the town of Welkom in Free State. The target population was male and female adult patients (18 years and above) on ART for a duration of 1 year or longer. Participants were selected by a random sampling of hospital case file numbers using random table numbers. The patients answered a set of 7 questions on symptoms, underwent weight and height measurements before having blood drawn for lactate assays Blood specimens for lactate assays were processed at the local National Health laboratory.