The ability of vitamin C supplement to influence lipid peroxidation and pulmonary function tests in healthy smokers was investigated. In this randomized double blind controlled trial, 56 smokers (S) received either 500 mg of vitamin C (C) or placebo (P) daily for 4 weeks. All completed the trial. Both groups were comparable and the number of cigarettes smoked were C: 14.2 ± 1.8 and P: 18.3 ± 2.0 pack-years. Plasma vitamin C concentrations increased significantly (p < 0.005) only in the group supplemented with vitamin C. Lipid peroxidation measured by breath pentane output (BPO) (C: 7.5± 1.4 vs P: 7.0 ± 1.3 pmol.kg^–1.min^–1) and plasma HPLC-separated malondialdehyde (MDA) (C: 0.58 ± 0.05 vs P: 0.47 ± 0.05 nmol.ml^–1) were not significantly different between the 2 groups at baseline and did not change after four weeks of vitamin C supplementation (BPO: C: 5.3 ± 0.9 vs P: 5.5 ± 0.9 pmol.kg^–1.min^–1; HPLC-MDA: C: 0.50 ± 0.07 vs P: 0.42 ± 0.07 nmol.ml^–1). No changes were detected in pulmonary function tests even in heavy smokers. Therefore, 4 week supplementation with 500 mg of vitamin C did not change lipid peroxidation indices and had no effect on pulmonary function tests.