The authors report the case of a female aged 40 who was subject to syncopal attacks, and had an apical end-systolic murmur. The presence of an idiopathic prolapse of the mitral valve was demonstrated, as well as the familial nature of the condition, and the fact that the syncopal attacks were caused by ventricular tachycardia, The case report is followed by a discussion of the cardiac arrhythmias which are likely to accompany this particular mitral lesion, the difficulties in treatment which arise, and finally the danger of sudden death by ventricular fibrillation inherent in this condition, a danger which it is stille difficult to quantify.