Ethanol is oxidized to acetaldehyde by alcohol dehydrogenase (ADH) and this is the main way of alcohol oxidizing in social drinkers. Microsomal Ethanol Oxidizing System (MEOS) is induced in ethanol addicted individuals and plays the main role in ethanol oxidation in this group. The aldehyde dehydrogenase (AIDH), enzyme oxidizing acetaldehyde to acetic acid, activity is not induced and does not get changed. This may lead to acetaldehyde cumulation in alcoholics. The aim of the study was to define the usefulness of acetaldehyde concentration measurement in the blood as the marker of alcohol addiction. 53 alcohol addicted and acutely poisoned patients (males) participated in the study. The control group was social drinkers, 43 males, accidentally poisoned with ethanol. Alcohol addiction was diagnosed according to ICD-10 criteria. Ethanol and acetaldehyde measurements were carry out on admission. The ethanol concentration in blood was measured by enzymatic method (ADH) and acetaldehyde concentration using headspace gas chromatography. Because of differences of ethanol concentrations in two examined groups (alcoholics and social drinkers) the normalization of acetaldehyde results was performed dividing acetaldehyde concentration by ethanol concentration. The results indicated higher blood acetaldehyde and ethanol concentrations in alcoholic individuals acutely poisoned with ethanol in comparison to acutely poisoned social drinkers. The acetaldehyde/ethanol ratio, used in the study, is higher in alcoholics as well and the difference is statistically significant. These results suggest the usefulness of acetaldehyde concentration measurement as a marker of ethanol addiction.