We examined a group of former oral contraceptive (OC) users, who had experienced cerebrovascular insufficiency, for the presence of hypercoagulable states. We found hypercoagulability in this group in the form of decreased plasma antithrombin III activity, increased platelet coagulant activity, and elevated plasma beta-thromboglobulin level. Certain characteristics (cigarette smoking, vascular headache, hyperlipidemia, and mitral valve prolapse) were encountered with increased frequency among former OC users who had experienced cerebrovascular insufficiency. The association of mitral valve prolapse with OC-related cerebrovascular insufficiency was particularly striking. We propose that preexisting hypercoagulable states, such as may exist in a subset of individuals with mitral valve prolapse, will magnify the risk of OC-related cerebrovascular morbidity.