The P-waves in standard electrocardiograms were analyzed in 101 normal children and adolescents and in 88 with asymptomatic rheumatic heart disease. Results were correlated with roentgenographically significant left atrial enlargement defined by new criteria that increase precision in x-ray assessment. Abnormalities in P-waves were more consistent in lead V-1 than in lead II. Left atrial enlargement could be suspected when a broadly-notched PIIor an inverted or diphasic PV-1was seen, and confirmed if the P-wave downstroke in V-1 was more than 1.0 mm deep or lasted more than 0.06 sec, or if PIIlasted more than 0.12 sec. P-waves were abnormal in lead V-1 for 29, and in lead II for 8, of 35 patients with significant left atrial enlargement. Normal and abnormal P-waves often overlapped in the analysis of lead II, but were sharply differentiated in lead V-1.