The mechanism of the electrocardiographic pattern termed left atrial enlargement was evaluated in 21 patients. Left atrial size and pressure as well as interatrial conduction were correlated with electrocardiographic left atrial enlargement using echocardiography, mean pulmonary capillary wedge pressure and activation time from the P wave to the coronary sinus. In the group as a whole only prolongation of interatrial conduction time was consistently related to the electrocardiographic pattern of left atrial enlargement; left atrial size or pressure was not predictably abnormal in patients with this pattern. Five patients had neither elevation of pulmonary capillary wedge pressure nor echocardiographic evidence of an enlarged left atrium. When the etiologic type of heart disease was analyzed, an enlarged left atrium correlated with electrocardiographic left atrial enlargement only in patients with rheumatic mitral valve disease (eight of nine patients). Elevated pulmonary capillary wedge pressure correlated with electrocardiographic left atrial enlargement in all four patients with cardiomyopathy. In patients with coronary artery disease the electrocardiographic pattern was unrelated to either left atrial pressure or volume overload. Thus, the electrocardiographic pattern termed left atrial enlargement appears to represent an interatrial conduction defect that can be produced by a variety of factors.