One hundred and eighty-two patients (100 females, 82 males) with mitral valve prolapse (MVP) confirmed by echocardiography are described. Their ages range from 12 to 87 years (mean 48 years). The symptoms of breathlessness, pain in the chest and palpitations were analysed. They were associated with left ventricular failure, co-existing ischaemic heart disease and arrhythmias in some, but in a proportion the symptoms were thought to be due to psychoneurosis. Seventy-two patients (40 per cent) were referred because of complications of MVP. In 67 patients (37 per cent) the condition was discovered by chance and in 43 patients (24 per cent) neurotic symptoms had led to referral to hospital. A systolic click was heard in 117 patients (54 per cent); 41 patients (23 per cent) had a late systolic murmur and 30 patients (16 per cent) had a pansystolic murmur. The incidence of murmurs rose with increasing age, and pansystolic murmurs were more frequent in males. Thirty-two patients (18 per cent) had neither a click nor a murmur. Twenty-four patients (13 per cent) had associated supraventricular tachycardia and 22 (12 per cent) atrial fibrillation. Twelve patients (7 per cent) had severe mitral incompetence and eight (4 per cent) developed bacterial endocarditis. Only three patients had symptoms suggesting cerebral ischaemia. Twelve patients (7 per cent) had associated aortic incompetence. Twenty-two patients had had an inguinal hernia, the incidence in males over 50 being 26 per cent. Twenty-six patients (14 per cent) had non-specific T wave changes in the electrocardiogram. Echocardiography showed that 112 patients (62 per cent) had mid-systolic buckling of the posterior leaflet and 70 patients (38 per cent) had holosystolic prolapse. In view of the high incidence of complications it is felt that the long-term prognosis not as good as has been generally believed.