Accessory mitral valve (AMV) is a rare cause of left ventricular outflow tract (LVOT) obstruction. This case report demonstrates the progression from a hypertrophic cardiomyopathy to a mid-cavity obstruction; abnormal mitral valve architecture, including AMV with increasing obstruction; progressive symptoms; and full documentation of details by transesophageal echocardiogram. The patient underwent a successful operation for removal of the AMV at age 21 years.
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