Differentiating between alcoholic and nonalcoholic hepatic steatosis is often a difficult clinical task. However, decreased fatty acid mitochondrial oxidation appears as the main factor for alcoholic steatosis, whereas nonalcoholic steatosis may be due to other causes. We studied mitochondrial function, based on a ¹³C-ketoisocaproic acid (¹³C-KIC) breath test, in nine alcoholic and 12 nonalcoholic steatosis patients and 10 healthy volunteers. Our results showed a 42% ¹³C-KIC decarboxylation decrease in alcoholic steatosis patients, but not in nonalcoholic steatosis patients. This noninvasive breath test appears helpful for the diagnostic work-up of hepatic steatosis.