There are few functional tests for liver function that selectively indicate the toxic effect of alcohol abuse. To explore the long-term impact of excessive alcohol use on the liver, there is a need for such specific analyses. The Ketoisocaproate breath test has been shown to be a specific marker of mitochondrial function in the liver, which is known to be selectively affected by heavy alcohol intake. This method was evaluated by analyzing 13 male patients with severe alcohol dependence and comparing these with 10 healthy volunteers, 5 women and 5 men. All alcoholic patients reported a heavy intake of alcohol during the last month before the test and all had some form of abnormal liver status as assessed by traditional markers such as aspartate aminotransferase, alanine aminotransferase, or γ-glutamyltransferase analyses. The results showed that healthy women had a higher percentage exhalation of 13CO2 than both healthy males and alcoholic males. In contrast to previous studies, we found no significant impairment of Ketoisocaproate decarboxylation as an effect of chronic intake of alcohol or alcohol-induced steatosis. The findings could not be explained completely by concurrent drug intake used in the routine detoxification of the patients. Thus, the value of the Ketoisocaproate breath test as a biological marker of excessive alcohol consumption appears to be limited because of a fast normalization of the values.