I studied the prevalence and symptoms of idiopathic mitral valve prolapse by auscultation in 972 consecutive patients in an adult general medical population. Forty-five patients (4.6%) had idiopathic mitral valve prolapse defined by a nonejection click with or without a late systolic murmur. The prevalence was not significantly different in men and women. The mean age (49.9 yr) and age distribution of patients with prolapse were similar to those of patients without prolapse (47.7 yr). The prevalence of dizziness (4.1% vs. 1.5%), fatigue (4.4% vs. 2.6%), and palpitations (4.4% vs. 1.3%), was not significantly greater in patients with or without prolapse. Atypical chest pain (13% vs. 4.3%) and chronic anxiety (8.8% vs. 2.9%) were more frequent (less than 0.05) in the patients with prolapse than in those without prolapse. Of the patients with prolapse, 29 were healthy without clinically identifiable diseases while 16 had medical diseases. In the group without prolapse, 184 patients were healthy and 707 had other diseases. When patients with isolated prolapse without other associated diseases were compared to healthy patients without prolapse, the prevalence of atypical chest pain (17.4% vs. 17.2%) and chronic anxiety (7.1% vs. 10.3%) were not significantly different. When patients with prolapse and other diseases were compared to patients without prolapse and other diseases, the prevalence of atypical chest pain (6.2% vs. 1.1%) and chronic anxiety (6.2% vs. 1.7%) was again not significantly different. Thirty-two patients without prolapse were suspected but not confirmed of having disease and were not included in this analysis. The results would have been unaltered by their inclusion in the diseased group without prolapse.