Mean platelet volume (MPV) is an indicator of platelet activation. We investigated the acute effect of passive smoking on MPV in healthy volunteers. Healthy nonsmoker volunteers (n = 90) spent 120 minutes in a smoke-free room. Then they were exposed to passive smoking in 3 groups consisting of 30 volunteers each in group A (carbon monoxide [ CO] < 7.5 ppm), group B (CO = 7.5-15 ppm), and group C (CO > 15 ppm) for 120 minutes in a smoking room. Mean platelet volume did not change in group A and in the smoke-free room. Mean platelet volume increased significantly at the 60th minute and continued to increase at the 90th and 120th minute in group B. Mean platelet volume increased significantly at the 30th minute and continued to increase at the 60th, 90th, and 120th minute in group C. In conclusion, acute exposure to passive smoking affected MPV according to the duration and intensity of smoke.