Standard 12 lead electrocardiograms (ECG) and timed Frank vectorcardiograms (VCG) were recorded in 53 subjects with atrial fibrillation. Thirty-eight patients had echocardiographically documented left atrial enlargement (greater than 4.0 cm) and 15 patients had normal atrial dimensions. The magnitude of the largest "f" wave component during an average cycle length was measured in lead V1 of the ECG and the horizontal plane VCG running loop. Relative sensitivies for detection of left atrial enlargement were: VCG, 25/38 (66%) and ECG 10/38 (26%). An enlarged left atrial internal dimension was diagnosed by the VCG alone in 21 of the 38 subjects (55%). In the group of 15 patients with normal echocardiographic left atrial internal dimensions the prevalence of ECG false positive diagnosis for enlarged left atrial size was 6% in contrast with 0% for the VCG. It is concluded that: 1) the timed Frank VCG is superior to the ECG for the detection of echocardiographically demonstrable left atrial enlargement; 2) the timed VCG and ECG represent complementary techniques for identifying patients with abnormally large left atria; and 3) large fibrillatory waves are rarely observed on the ECG or VCG when the left atrial internal dimension is echographically normal.