Various functional abnormalities of the autonomic nervous system have been reported in symptomatic patients with mitral valve prolapse. It has also been suggested that mitral valve prolapse may be a component of a neurovascular endocrine abnormality and a marker for anxiety. Eleven consecutive patients with mitral valve prolapse (six men and five women), five of whom were asymptomatic, were studied. In comparison with 11 control subjects matched for age and sex who underwent 60-degree upright tilt, there was no significant difference between plasma norepinephrine levels, heart rate, and blood pressure, before, during, and after tilting. Psychologic testing for anxiety neurosis in both groups showed no significant difference on any of these measurements. There was therefore no evidence of autonomic dysfunction or neurosis In the patients with mitral valve prolapse. This variance with the findings of other investigators is probably related to their study of patient groups skewed by a disproportionate number of symptomatic females; the patients in the study reported herein are more representative of mitral valve prolapse in the general population. The symptoms attributed to mitral valve prolapse are quite likely adrenergically mediated and precipitated by anxiety, but this probably represents a coincidence of two common conditions encountered in medical practice.