Although alcohol abuse is known to cause a wide array of adverse effects on blood cell formation, the molecular mechanisms by which alcohol exerts its toxic actions have remained poorly defined. Elevated mean corpuscular volume (MCV), macrocytosis, is the most typical morphological abnormality induced by excessive ethanol consumption. This paper reviews recent data indicating that acetaldehyde, the first metabolite of ethanol, may play a role in the haematological derangements in peripheral blood cells and in bone marrow of alcoholic patients. Studies in experimental animals and in human alcoholics have shown that acetaldehyde can bind to proteins and cellular constituents forming stable adducts. Elevated adduct levels have been found from the erythrocytes of alcohol abusers, which may also be associated with ethanol-induced effects in haematopoiesis and adverse consequences in cellular functions.