 The Yankees – Red Sox rivalry is a rivalry between the New York Yankees and Boston Red Sox baseball teams of Major League Baseball ( MLB ) . The two teams have competed in MLB 's American League ( AL ) for over 100 seasons and have since developed one of the fiercest rivalries in sports . In 1919 , Red Sox owner Harry Frazee sold star player Babe Ruth to the Yankees , which was followed by an 86 @-@ year period in which the Red Sox did not win a World Series . This led to the popularization of a superstition known as the " Curse of the Bambino " , which was one of the most well @-@ known aspects of the rivalry . 
