The identification of LA enlargement may have important clinical implications. Previous correlations of ECG P wave morphologies associated with LA enlargement and echocardiography have been limited by the use of small numbers of patients and by the employment of M-mode echocardiography without the benefit of two-dimensional guidance. The purpose of this study was to further examine the sensitivity and specificity of various P wave morphologies (P wave greater than or equal to 110 msec, notched P greater than or equal to 40 msec, and PTFV1 greater than or equal to 40 msec.mm) for the diagnosis of LA enlargement and to determine if these waveforms may be predictive of LA size. ECGs and surface echocardiograms obtained within 1 week of each other were evaluated in 551 patients (140 normal and 411 study subjects). The various P wave morphologies were found to be poorly sensitive (30% to 60%) but very specific (90%) for LA enlargement. Combinations of P wave morphologies did not improve sensitivity or specificity. ECG features did give an estimate of the degree of LA enlargement. When PTFV1 is greater than or equal to 40 msec.mm, 95% of patients had LA size greater than or equal to 40 mm; and when this parameter was greater than or equal to 60 msec.mm, 75% had LA size greater than or equal to 60 mm. These criteria for LA enlargement on the ECG are specific and predictive of the degree of LA enlargement measured by echocardiography.