Thromboembolism in mitral valve prolapse (MVP) has been suggested to occur in relation to valvular cul-de-sac recesses and endothelial wear-and-tear denudations. Such pathology would be suspected to be prominent in advanced cases of MVP necessitating open-heart operation. We therefore analysed the prevalence of perivalvular thrombi and episodes of systemic arterial embolism (SE) prior to operation in 21 consecutive patients with MVP and severe valve regurgitation. No perivalvular or left atrial thrombi were found during open-heart surgery, but three of the patients (14%, 3-36% with 95% confidence limits) experienced SE, in one case despite long-term anticoagulation. All cases of SE took place less than 3 months before operation. The results indicate that persistent thrombi are rare in MVP and severe valve regurgitation. However, such patients run a substantial risk of SE when they reach a clinical and haemodynamic stage that makes valve surgery mandatory.