OBJECTIVES: The systolic anterior motion (SAM) of mitral valves occurs at a certain rate despite the introduction of several preventive procedures. The purpose of this study was to investigate its mechanism by analysing the change in mitral valve morphology associated with operative procedures. METHODS: Components of mitral valves were measured before and after operative procedures by transoesophageal echocardiography in 179 patients who underwent mitral valve repair. Comparisons were made between 15 patients with SAM (SAM group) and 164 patients without SAM (non-SAM group). RESULTS: Morphological analysis in all the studied patients revealed that operative procedures shifted the coaptation point towards the left ventricular outflow tract by 6.9 mm and increased the extra portion of anterior leaflet that extended beyond the coaptation point by 5.4 mm. These changes were enhanced in the SAM group. Intergroup comparison revealed that there were no differences in the preoperative mitral valve morphologies between the two groups. After operative procedures, however, the SAM group showed smaller annular diameter and smaller coapted anterior/posterior length ratio compared with the non-SAM group. CONCLUSIONS: The results of this study show that operative procedures might modify the morphology of mitral valves susceptible to developing SAM. Postoperative smaller annular diameter and anterior shift of coaptation point were considered to contribute to the development of SAM.