Cigarette smoking is associated with oxidative stress and increased risk of many chronic diseased. Smoking inducts depletion of cellular antioxidant and is also known to be associated with an increased homocysteine level. Exposure to tobacco smoke has negative impact on the folic acid level. Folic acid is cofactor by demethyla-tion of homocysteine to non toxic methionine. The aim of this study was to determine the concentration of total plasma homocysteine and 5-methyltetrahydrofolic acid in active and non smokers and to evaluate the influence of the tobacco smoke exposure on urinary cotinine levels. The results indicated significant increase of total plasma homocysteine in smokers, than non smokers. We also investigated the negative influence of tobacco smoke on the level of folic acid in plasma and it suggests, that additional supplementation of folic acid in smokers is necessary.