Summary. While an estimated 13% of women with unexplained menorrhagia have von Willebrand disease (VWD), the frequency of other potential bleeding disorders has been uncertain. This study describes the relatively wide range of laboratory characteristics of women with unexplained menorrhagia and presents issues affecting diagnosis in this population. Women with pictorial blood assessment chart (PBAC) score >100 were identified at six U.S. sites and asked to remain drug free for 10 days prior to testing. Blood was collected on one of the first four menstrual cycle days and tested at a central laboratory for procoagulant factors, VWD and fibrinolytic factors. Platelet function testing by PFA-100® (PFA) and platelet aggregation with ATP release (PAGG/ATPR) were performed locally using standardized methods. Among 232 subjects, a laboratory abnormality was found in 170 (73.3%), including 124 of 182 White (68.1%) and 34 of 37 Black (91.9%) subjects; 6.0% had VWD, 56.0% had abnormal PAGG/ATPR, 4.7% had a non-VWD coagulation defect (NVCD) and 6.5% had an abnormal PFA only. AGG/ATPR was reduced in 58.9% of subjects, with multiple agonists in 28.6%, a single agonist in 6.1% and ristocetin alone in 24.2%. Frequencies of PAGG/ATPR defects varied by study site and race; frequencies of VWD and NVCD were similar. Laboratory abnormalities of haemostasis, especially platelet function defects, were common among women with unexplained menorrhagia across multiple U.S. sites. To what degree these abnormalities are clinically significant requires further study.