Introduction: Smoking is well known as a risk factor of arteriosclerotic disease and for effects on blood fluidity. The chronic effects of smoking on hematology are the increase of WBC and hematocrit. Chronic smoking causes a decrease of blood fluidity, but the acute effect of smoking on blood fluidity is not well known. In this study, we examined the changes of blood fluidity before and after smoking to estimate the acute effect of smoking on blood fluidity. Subject: The subjects were 14 young men (7 controls; age 23.4±2.3 year, 7 smokers; age 23.3±2.2 year,) who don’t have internal disease. We obtained informed consent from them. Method: The blood collections were done before smoking, right after smoking and 5 minutes after smoking. The subjects quit smoking at least six hours before blood collection. The blood was drawn from the elbow nasal venules of retina. We used a microchannel array flow analyzer (MC-FAN). The MC-FAN utilizes 8736 micro channels fabricated in silicon, simulates capillary vessels in vivo and is also an imaging instrument to visualize the state of blood flow. The whole blood passage time of 100 micro liters was measured to evaluate blood fluidity. The platelet rich plasma passage time of 100 micro liters was measured to evaluate platelet function. An automatic cell counter (Beckmancoulter Co., Ltd.) was used to measure red blood cells, white blood cells, hemoglobin, hematocrit, and platelets. Result: The whole blood passage time and the platelet rich plasma passage time did not show significant differences in the two groups. In the smokers, the whole blood passage time measured “right after smoking” (48.0±5.0 sec) was significantly longer than that of “before smoking” (45.5±4.4 sec) and “5 minutes after smoking” (44.4±3.5sec) almost returned “before smoking”. In the smokers, the platelet rich plasma passage time did not show significant differences on the time course. Discussion: There were no significant differences between smokers and non-smoker on the blood fluidity and blood cell counts. The whole blood passage time of “right after smoking” was significantly longer than that of “before smoking”. The blood cell counts of “right after smoking” showed a tendency to increase.