To test the hypothesis that orthostatic hypotension could represent an alternative mechanism contributing to the symptoms of mitral valve prolapse, the systolic and diastolic arterial blood pressures were measured in the supine and standing positions in 86 patients with the diagnosis confirmed by echocardiography. Orthostatic hypotension was demonstrated in 12 patients. Ten of them presented with a history of recurrent lightheadedness, dizziness or syncope and constitute 59 percent of the total number of patients with such symptoms in this series. Although nine of these 10 patients reported transient lightheadedness or dizziness during periods of ambulatory electrocardiographic recording, in only one were the symptoms chronologically related to cardiac arrhythmias. On the other hand, eight of them described lightheadedness and two experienced near-syncope during the postural test in association with the orthostatic drop in blood pressure. Improvement in symptoms and correction of the orthostatic hypotension were demonstrated in seven patients after beta-adrenergic blockade with propranolol. Before therapy, the mean systolic blood pressure dropped from 114 +/- 3 mm Hg in the supine position to 78 +/- 1 mm Hg upon standing (p less than 0.001). In repeated postural tests performed after four weeks of treatment, the systolic blood pressure changed from 120 +/- 3 mm Hg supine to 115 +/- 1 mm Hg upon standing (p greater than 0.01). We conclude that orthostatic hypotension is a commonly unrecognized mechanism responsible for some of the symptoms of mitral valve prolapse, particularly in patients affected by recurrent lightheadedness, dizziness or syncope.