Recent studies have demonstrated that smoking may be one of the most significant risk factors in the development and progression of periodontal disease. Reports have indicated that smoking causes gingival blood flow to be decreased. However, studies on the effects of smoking on gingival blood flow have yielded contradictory results. The purpose of the present study was to determine the effect of smoking on gingival blood flow. One hundred volunteers(fifty non-smokers and fifty smokers) with good general and periodontal health, aged twenties(non-smoker : 22-29 years, mean=25.36, smoker : 23-29 years, mean=26.64) were selected. Laser Doppler flowmetry (floLAB, Moor Instruments Ltd., England) was applied to measure the gingival blood flow of interdental papilla, marginal gingiva, attached gingiva and alveolar mucosa of left and right upper lateral incisors. In smokers, following an overnight abstinence from smoking, gingival blood flow was measured before smoking, immediately after smoking, 1-, 2-, 3-, 4-, 5- and 6- hour after smoking from 9 a.m. to 3:30 p.m. The difference of blood flow in each tissue of non-smokers and that of each measuring time and each tissue of smokers were statistically analyzed by one way ANOVA and Tukey test. And the difference of blood flow between smokers and nonsmokers in each tissue was statistically analyzed by t-test. The results were as follows : 1. Mean blood flow was highest in alveolar mucosa, followed by interdental papilla, attached gingiva and marginal gingiva in both smokers and nonsmokers. There was a statistically significant difference in each tissue(p<0.05) . 2. There was no consistent result between mean blood flow before smoking in smokers and that of nonsmokers in each tissue. 3. There was a statistically significant difference between gingival blood flow at measuring time point and gingival blood flow of smokers in each tissue(p<0.05). The present study suggested that smoking could alter the gingival blood flow, thus might be partly contributed to periodontal destruction.