Unsuspected mitral stenosis. PURPOSE AND PATIENTS AND METHODS: We observed a series of patients in whom the diagnosis of mitral stenosis was first discovered in the echocardiography laboratory. Because of this experience, we examined the records of 152 patients with echocardiographic evidence of rheumatic mitral stenosis to determine the clinical characteristics and course of patients with unsuspected mitral stenosis as well as those factors that may have obscured the diagnosis. RESULTS: Of these 152 patients, 18 had mitral stenosis that was unsuspected clinically until the echocardiogram. These patients were elderly, with a median age of 72 years. They were all referred for echocardiography because of cardiac symptoms. Eight patients were referred for evaluation of congestive heart failure. Five patients were referred for evaluation of aortic valve disease. Three patients were referred because of cerebrovascular accidents and atrial fibrillation. The Doppler-determined mean diastolic mitral gradient ranged from 4 to 15 mm Hg (mean: 7 mm Hg). Mitral stenosis ranged in severity from trivial to very severe. Eight patients had moderate to severe mitral stenosis with estimated mitral valve areas less than or equal to 1.5 cm2. Seven had mild or trivial mitral stenosis with estimated mitral valve areas greater than 1.5 cm2. After further evaluation, two patients underwent mitral valve surgery with improvement of congestive failure. In three patients, warfarin therapy was begun to prevent emboli. Thus, five of 18 patients had a significant immediate change in therapy because of the discovery of mitral stenosis. CONCLUSION: The diagnosis of mitral stenosis may not be suspected in the presence of advanced age, other serious cardiac and medical conditions, or mechanical factors that complicate the physical examination. In these patients, mitral stenosis may be hemodynamically significant and may cause significant symptoms. 