Abstract.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 2 pM). To detect the effect of sex hormones, platelets were incubated with estradiol (10 nM) or testosterone (40 nM) for 30 min, then platelet aggregation studies were performed.LT method revealed that female had greater aggregability than male. With weak stimuli (<_ 1,iM 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 (<-1 reM), but that both estradiol (10 nM) and testosterone (40 nM) 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. has life-threatening bleeding during perinatal period. Some studies, however, have pointed out that the results in aggregometry between female and male are controversial [3,4]. The recently developed light scattering (LS) method has greater sensitivity than conventional LT method [5], since the LT method cannot detect small aggregates with less than 100 platelets, whereas the LS method can detect aggregates as small as two or three platelets [b, 7] . We applied this new method to exploring the sex differences in platelet aggregation. Sex hormones are the major difference between female and male, and the effect of estrogen on coagulation is sometimes discussed, but controversy still remains. High estrogen level is associated with