Myxoma is a non-cancerous (benign) tumor of the heart. In fact, it is the most common primary heart tumor. A primary heart tumor is one that originates within the heart as opposed to another organ or tissue. Approximately 75% of myxomas are located within the left atrium. Typically, myxomas grow off of the inter-atrial septum (wall that separates the two atria). Myxomas tend to be solitary (occur as a single mass).