The purpose of this study was to determine whether cigarette smoking acutely increased circulating platelet aggregates as detected by lowering of the platelet aggregate ratio. Eighteen (11 male and 7 female) volunteers ranging from 16 to 47 years of age rested during a 20-minute control period and smoked two unfiltered tobacco cigarettes during another 20-minute period. Platelet aggregate ratios (mean ± SD) before and after the control period were 0.86 ± 0.13 and 0.82 ± 0.12 respectively (p > 0.05) and before and after smoking were 0.84 ± 0.14 and 0.66 ± 0.18 respectively (p < 0.01). The mean plasma nonesterified fatty acid concentration was not significantly different before and after smoking suggesting that the decrease in platelet aggregate ratio was not mediated through elevation of the plasma nonesterified fatty acid concentration. Our results are consistent with the theory that platelet aggregates contribute to the increased incidence of myocardial infarction and sudden death from coronary artery disease observed among cigarette smokers.