The first open heart operation ever performed was by Clarence Dennis on April 5, 1951, at the University of Minnesota. For several years he had worked developing a heart-lung machine. The operation was on a young girl considered to have an ostium secundum atrial septal defect, but at the procedure a large ostium primum was identified instead. It could not be repaired, and the child died. The anatomic details of this heart have not been previously presented. For historical purposes, the clinical history, intraoperative course, and pathologic details will be discussed and shown.