A 30-yr-old man with acquired immunodeficiency syndrome treated with zidovudine developed biopsy-proven mitochondrial myopathy. Chronic lactic acidosis (lactate, 10 +/- 1 mmol/L) persisted for more than 5 wk. Liver function tests were normal, but the concentration of lactose rose to 16.1 mmol/L when 500 mmol of ethanol was infused. The concentration of lactose rose by only 1.5 mmol/L with maximally tolerated exercise. If this mitochondrial lesion compromised flux through the electron transport system, increased turnover of ATP with exercise should have exacerbated the degree of lactic acidosis because of increased need to regenerate ATP via glycolysis. Two possible explanations will be discussed: first, there was both a rapid rate of production of lactic acid in affected muscles in conjunction and an equally rapid rate of removal by uninvolved organs. Second, there was a low net rate of production of lactic acid in involved muscles despite the exercise.