Serum malanodialdehyde (MDA) levels in 40 smokers and 23 non-smokers belonging to different age groups were estimated. The MDA levels were high in smokers of all age groups having a history of smoking for less than 10 years. MDA levels, however, were not elevated in smokers with a history of smoking for more than 10 years. These results are discussed and are interpreted as suggestive that MDA might alter the LDL and lead to deposition of cholesterol in arterial wall macrophages explaining thereby the risk of ischaemic heart disease in cigarette smokers.