Systemic embolism is a potential and severe complication of percutaneous mitral valve dilatation. In our series of 80 cases, the incidence of systemic embolism was 3.75% (3 cases). Two cases occurred during the procedure itself. The cause of embolism was different in each case: cruoric thrombus formation occurred in 2 cases during or after dilatation, and was attributable to inappropriate heparinization, and catheter and guidewire thrombogenicity. In both cases heparin administration was delayed or given at a lower dosage. In the third case, calcific embolism occurred several days after valve dilatation. We think that the embolic calcified fragment was detached from the mitral leaflet at the time of or after balloon inflation. In these 3 cases, intraatrial thrombus mobilization was not the mechanism of systemic embolism. In 2 cases, transesophageal echocardiography had been performed before dilatation and excluded the presence of an atrial thrombus the day before the procedure. It is concluded that, together with mobilisation of left atrial thrombi, which can be adequately detected by transesophageal echocardiography, catheter-induced thrombi represent a significant cause of embolic complications and must be prevented by giving full-dose heparin during the total duration of the procedure. Calcific embolism may also occur, and may become more frequent if mitral valve balloon dilatation is proposed to a larger number of patients with valvular and subvalvular calcifications.