Earlier studies in platelet aggregation have shown that females seemed to have greater aggregability than males as detected by conventional aggregometry which used light transmission (LT), but controversy still remains. This study was performed to determine whether sex difference exists in platelet aggregation by using the recently developed laser light scattering (LS) method, which can detect small aggregates (i.e., two or three platelets). Blood was drawn from healthy volunteers (10 male and 10 female in follicular phase after menstruation), and platelet aggregation was detected by either LT or LS method in platelet rich plasma. Platelet aggregation was stimulated by increasing concentration of adenosine 5' diphosphate (ADP, 0, 0.5, 1 and 2μM). To detect the effect of sex hormones, platelets were incubated with estradiol (10nM) or testosterone (40nM) for 30min, then platelet aggregation studies were performed. LT method revealed that female had greater aggregability than male. With weak stimuli (_??_1μM ADP), LS method showed that females had more medium aggregates than males, and that testosterone decreased small aggregates, and that estradiol decreased all sizes of aggregates. These data suggest that the female is more conductive to platelet aggregation than the male at a physiologic concentration of ADP (_??_1μM), but that both estradiol (10nM) and testosterone (40nM) have countereffects on platelet aggregation at the same condition. Therefore, the reason why females have greater aggregability than males may partly be explained by their lack of testosterone, but the mechanism still remains to be elucidated.