OBJECTIVE To compare the accuracy of left atrial enlargement (LAE) diagnosis made by electrocardiographic criteria with those obtained using M-mode echocardiography. METHODS We studied 273 patients age 17 to 87 (mean 49) years, 115 men, white 95.5%, black 3.5% mulattos 1%, with or without heart disease of different etiologies. The ECG criteria studied were: a) P terminal force in V1 > or = 0.04 mmx s; b) Pt force in V1 duration > 0.04s; c) Pt force in V1 depth > or = 1 mm; d) P wave notching in D2 with interpeak distance > or = 40ms; f) presence of atrial fibrillation. The gold-standard for LAE was left atrial dimension > 40 mm obtained by echocardiography. RESULTS The percentage of correct diagnosis were: atrial fibrillation (88%), Morris index (75%), PtfV1 negativity > or = 1 mm (74%), notched P wave in D2 with interpeak distance > or = 0.04 s (70%), PtfV1 with duration > 0.04 s (64%) and P wave duration in D2 > 0.11s (46%). CONCLUSIONS Conventional ECG has limited value for detecting LAE. A higher correlation was found between atrial fibrillation and changes in P wave in V1 and the echocardiographic LAE.