Sensitivity, specificity and predictive accuracy of 5 electrocardiographic criteria for chronic myocardial infarction (MI) in the presence of left bundle branch block (LBBB) were evaluated in 47 patients with known (clinical and electrocardiographic Q wave) MI and 28 patients without MI. Two right ventricular sites were paced (producing LBBB with normal or left axis). The effect of infarct location on these criteria was also evaluated. In patients with LBBB: Cabrera's sign (notching of the upstroke of S wave in V3,4,5) is seen more often with MI than without (anterior more often than inferior), and the left axis increased its sensitivity. Chapman's sign (notching of upstroke of the R wave in I, L or V6) is more common in patients without MI, and its specificity and sensitivity are not altered by axis. Sensitivity of notching in 2, 3, F is too low to be clinically useful. Q 2, 3, F indicates only left axis; predictive accuracy for MI is high (100%) only for normal axis where sensitivity is low (3%). (5) Q 1, L, V6 is no more frequent with MI than without, but if MI is present it is more common in anterior than inferior infarction. Specificity and predictive accuracy are too low to be clinically useful as indicators of MI.