The effect of coffee drinking on platelet reactivity was studied in 12 healthy subjects. Plasma beta-thromboglobulin concentration was determined before and one hour after administration of 100 mg of caffeine, corresponding to one cup of coffee. Mean values were 47.0 +/- 19.3 and 179.3 +/- 85.5 ng/ml before and after caffeine administration respectively. The increase, 298 +/- 150%, is highly significant (p less than 0.001).