Objectives: To investigate the effects of tobacco smoking on serum vitamin B 12 , folic acid and haematological parameters in healthy Thai smokers and non-smokers. Design: Cross-sectional study of smokers and non-smokers in a military unit in Bangkok, Thailand. Setting: A military unit in Thailand. Subjects: One hundred and twenty-three male smokers from a military unit in Bangkok, who participated voluntarily in the study, were investigated. Sixty-six male non-smokers from the same unit were selected as controls. Fasting blood samples were collected for investigation of vitamin B 12 , folic acid and haematological variables. Results: The serum folic acid concentration of smokers was lower than that of nonsmokers, but was not statistically significantly different. Haemoglobin was lower in smokers than in non-smokers; 16.3% of smokers were anaemic compared with only 3.0% of non-smokers. Anaemia was not related to folate deficiency. The white blood cell count was found to be higher in smokers than in non-smokers. Conclusion:The results of this study suggest that there were low serum folic acid concentrations in smokers compared with non-smokers, which might contribute to the development of vascular and cardiovascular diseases. The higher white blood cell count might be indicative alterations in the immune functions of smokers.