['Mitochondrial toxicity is an important toxicity of antiretroviral therapy and may manifest as elevated blood lactate. Macrocytosis has been hypothesized to act as a marker of mitochondrial toxicity in persons with HIV infection. As part of a larger study on markers of mitochondrial toxicity we evaluated the relationship between resting lactate and erythrocyte mean corpuscular volume (MCV).', 'We studied the effect of micronutrient supplementation on lactate metabolism in three groups: those with HIV on and not on dideoxynucleoside-containing antiretroviral therapy and those without HIV. As part of the study we measured resting lactate and erythrocyte MCV after a 14-h fast.', 'Erythrocyte MCV was significantly higher among the 11 participants on antiviral therapy (109.3 femtoliters (fl)) compared to five controls without HIV (89.3 fl) and four controls with HIV not on antiviral therapy (88.3 fl). In addition a strong predictive relationship was seen between MCV and resting lactate (R(2)=0.548, p<0.01).'] Is macrocytosis a predictor of resting lactate concentrations in persons on dideoxynucleoside therapy for HIV infection?