BACKGROUND: Vitamin C has been reported to have a role in the decrease of airway hyperresponsiveness in animal models. This data is based on some metabolic actions of vitamin C, such as promotion of histamine degradation, producing mote PGE2 than PGE2 alpha in cyclooxygenase pathway, decrease of smooth muscle contraction, and acting as reducing agent of oxidant. It has been also known that heavy smokers have lower blood levels of vitamin C than nonsmokers and this deficiency in heavy smokers have been explained by several mechanisms, such as increased oxidation by oxidants and free radicals, increased biosynthesis of catecholamine and serotorim released by nicotine, and inadequate dietary intake. In this study, We attempted to assess effect of vitamin C on bronchial hyperresponsiveness in heavy smokers who have bronchial hyerresponsiveness and role of vitamin C on bronchial hyperresponsiveness. METHOD: To assess acute effect of vitamin C on airway hyperresponsiveness, blood sample for vitamin C level and spirometry, methacholine challenge test were done in 17 smokers and 8 nonsmokers, and one hour after oral administration of vitamin C 3 g, blood sample for vitamin C level and spirometry, metliacholine challenge test were repeated. To assess chronic effect of vitamin C on airway hyperreeponsiveness after daily administration of vitamin C 1 g for one week in 17 smokers, blood sample for vitamin C level and spirometry, methacholirie challenge test were done. To assess role of vitamin C, after oral administration of vitamin C 3 g plus indomethacin 100 mg in 12 of 15 smokers who were reactive to methacholine challenge test, spirometry and methacholine challenge test were done and after oral intake of indomethacin 100 mg in 12 smokers who were reactive to methacholine challenge test, spirometry and metbachoine challenge test were repeated. RESULT: There were no significant differences in whole blood vitamin C levels between smokers(1.17+/-0.22mg/dL) and nonsinokers(1.14+/-0.19 mg/dL) (p>0.05). Fifteen of the 17 smokers(88.2%) were reactive to metbacholine chaflenge test amd 10 of the 15 smokers who were reactive to methacholine challenge test were less than 8 mg/dL in PC20FEV1, and 7 of the 8 nonsmokers(87.5%) were nonreactive to methacltoline challenge test There were significant decrease in bronchial responsiveness after oral administration of xitamin C 3 g in 13 of the 15 smokers who were reactive to methachoine challenge test. This significant decrease persisted with maintenance daily administration of 1 g for one week. PC20FEV1 were not correlated to vitamin C levels in smokers. After oral administration of indomethacin 100 mg, significant reduction of bronchial responsiveness that occured after oral administration of xdtamin C 3 g in smokers were attenuated. CONCLUISON: Although there were no significant differences in whole blood vitamin C levels between smokers and nonsmokers, heavy smokers have significant increase in bronchial responsiveness than nonsmokers. This bronchial hyperresponsiveness of heavy smokers can be attenuated by vitamin C supplement Disappearance of vitamin C effect by indcrnethaein supplement may suggest that vitamin C exert its effect via alteration of arachidonic acid metabolism.