The use of antiplatelet therapies decreases the incidence of mortality in persons prone to cardiovascular events. Several in vitro studies suggest that garlic may decrease platelet aggregation. We aimed to test the acute effects of garlic on platelet aggregation in 14 healthy volunteers using a randomised, double-blind, placebo-controlled, crossover research method. The active agent tested was solvent-extracted garlic oil incubated in ethanol to obtain organosulphur compounds that demonstrate the highest antiplatelet activity when tested in vitro. Platelet aggregation was induced ex vivo by adrenaline, collagen or adenosine diphosphate (ADP). Four hours after consuming one large dose of oil derived from 9.9 g garlic, there was little or no effect in the reduction of platelet aggregation. Platelet aggregation induced by adrenaline was reduced slightly but significantly (P<0.05; 12% reduction). The oil had no effect on collagen- or ADP-induced aggregation. The results of this controlled trial indicate that this type of garlic oil should not be relied on in persons with conditions in which reductions in platelet aggregation are desired or necessary.