Background: Moderate alcohol consumption reduces the risk of cardiovascular diseases, especially coronary heart disease (CHD). Because of the presence of polyphenols in red wine, this type of beverage may be superior to other alcoholic drinks in the prevention of CHD. Inhibition of platelet aggregation is thought to be one of the mechanisms underlying this favorable effect. The present study analyzes the direct effect of alcohol and red wine polyphenols on platelet aggregation. Methods: Unfractionated red wine, a red wine polyphenolic extract, and alcohol were added in different concentrations to a standardized quantity of blood platelets 2 min before aggregation was induced by different concentrations of ADP. Aggregation was measured in an aggregometer and results were compared to a control 0.9% NaCl solution. Results: Alcohol in concentrations up to 0.24 percent did not inhibit platelet aggregation in vitro initiated with ADP The polyphenolic red wine extract inhibited aggregation dose-dependently and significantly from concentrations of 45 mg/l ( [Formula: see text] ) or more. Red wine only inhibited platelet aggregation at very high concentrations ( approximately 0.24 and 0.48 alcohol%). Conclusions: Consumption of red wine has an inhibitory effect on platelet aggregation, which is caused by the polyphenolic compounds in the wine. Alcohol itself does not have a direct inhibitory effect within a range up to 0.24 percent. Since this effect is only observed at very high concentrations, it is unlikely to be of clinical relevance in a moderate drinking pattern. The results do not exclude platelet inhibition by wine in vivo. However, this must be related to metabolic changes rather than to direct blockade.