OBJECTIVE Non-alcoholic liver disease (NAFLD) is one of the most common causes of chronic liver disease, and its prevalence and medical importance is increasing worldwide. Changes in enzyme activity in liver cells in various liver diseases are reflected by an increase in serum enzymatic activity. For example, alcohol dehydrogenase activity (ADH) and aldehyde dehydrogenase (ALDH), that occur in the liver in large quantities, correlate with disease severity during cirrhosis. In the current study, the activity of ADH isoenzymes and ALDH in the serum of patients with NAFLD was investigated. METHODS Serum samples were collected for routine biochemical studies from 55 patients with NAFLD patients and from 50 healthy individuals. Class I and II ADH and ALDH activity were measured by spectrofluorometric method. Photometric methods were used to measure ADH class III, IV and total ADH activity. RESULTS Total ADH activity was significantly higher in non-alcoholic fatty liver (NAFL) and non-alcoholic steatohepatitis (NASH) than in healthy individuals (44 and 48.5% activity, respectively). The median total activity of ADH was 1,164 mU/l in patients with NAFLD, 1,258 mU/l in NASH and 648 mU/l in the control group. The increase in ADH class I and II isoenzyme in serum of patients with NAFL and NASH was statistically significant. The activity of ADH I, ADH II, and total ADH significantly increased with increasing disease progression. CONCLUSIONS The activity of isozymes of class I and II alcohol dehydrogenase in patients with NAFLD is enhanced and appears to be due to the release of these isoenzymes from damaged hepatocytes.