A prospective long-term follow-up study was made of 300 patients with idiopathic mitral valve prolapse, diagnosed by clinical, cineangiographic and echocardiographic criteria. There were 136 male and 164 female patients, ranging in age from 10 to 87 years (mean 42.2). The study included all patients with primary mitral valve prolapse, irrespective of clinical condition at the onset, and excluded only those patients with "secondary" mitral valve prolapse attributable to an accompanying established disorder. The average follow-up period was 6.1 years (range 6 months to 20 years). Two patients died of a noncardiac cause. The clinical condition of 153 patients remained stable. In 27 patients a supraventricular tachycardia occurred that was readily controlled with medication and caused no serious clinical complications. In 20 patients signs of mitral regurgitation appeared, but the patients remained clinically asymptomatic. Serious complications developed in 100 patients. Sudden death, most likely due to ventricular fibrillation, occurred in three patients; documented ventricular fibrillation was seen in two. Ventricular tachycardia developed in 56 patients, but in all instances the rhythm disorder was managed effectively and durably with medication. Infective endocarditis occurred in 18 patients, 4 of whom died during treatment and 6 of whom needed mitral valve replacement. The remaining eight patients suffer from severe mitral regurgitation that will require surgery in the near future. Twenty-eight patients underwent mitral valve operation because of progressive regurgitation. Cerebrovascular accidents occurred in 11 patients, but lifelong treatment with coumarin derivatives or antiplatelet aggregation agents was not considered necessary.(ABSTRACT TRUNCATED AT 250 WORDS)