SummaryA comparative study of coagulation and fibrinolytic laboratory parameters was undertaken in four countries (Salvador, Brazil; Singapore; Santiago, Chile and Dublin, Ireland) among apparently healthy women of reproductive age. A continuous external quality control scheme of the laboratory measurements was employed to permit comparison among centres. Significant and consistent differences were found between the four centres. In Dublin, the prothrombin time and activated partial thromboplastin time (APTT) were accelerated, and the specific factor assays showed more activity, whereas the antiprotease levels were higher than in the other centres. In Salvador, a contrasting tendency was found with longer prothrombin times and APTT and lower Factor VII and antiprotease levels. The results from the other two centres were approximately midway between these two extremes. The study has revealed important differences in the coagulation and haemostatic tests between women from widely diverse geographical areas. It is not certain whether these are due to ethnic, nutritional or economic factors but they may be related to the apparent varying incidence of thrombosis in these ethnic groups.