M-mode echocardiography was performed in 81 patients with chronic arterial hypertension in order to determine the specificity and sensitivity of the various ECG criteria used for diagnosing left ventricular hypertrophy (LVH) in the determination of left ventricular wall thickening (LVWT). Fifteen popular ECG criteria were studied and showed to be highly specific for LVWT (90 percent to 100 percent). TV1 greater than TV6, RV8 greater than 20 mm and SV1 + Rmax V5 or V6 greater than 35 mm were the most sensitive criteria (69 percent, 54 percent, and 52 percent respectively). The popular limb lead criteria for LVH were less sensitive than the precordial lead criteria in the determination of LVWT. The Estes point system, although less sensitive than some of the other voltage criteria, showed an absolute specificity for LVWT. The ST segment deviation with strain pattern was found in 46 percent of patients with LVWT. An isolated ST segment deviation without any other voltage criterion reflected at most only mild wall thickening. Six patients with LVWT had normal ECG; all of them were categorized in the mild LVWT group. Left axis deviation was found to be a poor indicator of wall thickening in uncomplicated hypertensive patients.