NRTI-related mitochondrial toxicity has been implicated in some fatal cases of lactic acidosis and liver failure. In addition, some investigators believe NRTI-related mitochondrial toxicity to be the culprit in the development of peripheral neuropathy and lipodystrophy in HIV-infected patients. There is a need for a sensitive, reproducible, and noninvasive marker of mitochondrial dysfunction. To date, the only available noninvasive marker is lactate, but lactate testing is insensitive and the significance and reproducibility of lactate levels in the HIV-infected population are questionable. Spectroscopy promises to be a very useful alternative for the evaluation of the in vivo effect of NRTIs on mitochondrial function.