The relationship between cigarette smoking and the respiratory function test of adolescents has been studied previously. Gold et al.11)found that FEV1/FVC decreased among adolescent smokers. Smoking habits and the number of cigarettes smoked per day were associated with the reduction in FEF25–75. Because our subjects were youths with no apparent respiratory pathology, we did not expect to find advanced impairment of lung function. Indeed, the vast majority of youths demonstrated respiratory function values within the normal range. Additionally, the low level of nicotine dependence, and the intensity and duration of smoking in our youth group were unlikely to cause the intense respiratory health effects that are usually observed in elderly smokers. In this study, FVC of the non-smoker group was significantly greater than that of the smoker group, while there was no significant difference in FEV1 between the groups. This result suggests that cigarette smoking affects the lung capacity of youth smokers, making the volume that is associated with the FVC test smaller than that of non-smokers. The reduction in FVC of smoker may be explained by the reduction in strength of the respiratory muscles. The results for FVC may have been influenced by the instructions given to subjects, to perform maximal inhalation and then perform maximal exhalation as rapidly and as completely as possible. Hence, the FVC test relies on the strength of respiratory muscles. Cigarette smoking affects the respiratory muscles through the influence of free radicals on the vascular system 16), leading to a reduction in respiratory muscle blood supply which adversely impacts respiratory function.