Early after acute myocardial infarction (AMI) residual viability can be detected by 2-D echocardiography during low-dose dobutamine (DOB) infusion. Aim of this study was to evaluate the relationship between signal averaged ECG (SAE) and the amount of viable myocardial tissue in patients with recent MI. Forty-eight pts, 37 males, mean age 60 +/- 8 years, underwent a DOB-echo 14 +/- 3 days after MI. SAE was recorded immediately before the test. An abnormal SAE was defined as having two of the following: fQRS =< 115 ms, LAS40 =< 38 ms, RMS40 =< 25 μV (40 Hz Butterworth filter; noise < 0.7 μV). DOB was intravenous infused at 5 and 10 mg/Kg/min, each dose during 5'. Optimal echo recordings were obtained in 42 pts. Quantitative analysis of the LV, divided in 24 segments, was performed on digitized images and % area change (%AC) from peak diastolic to peak systolic area calculated by centerline ''fixed'' system. Of 1008 segments, 208 were judged asynergic (%AC < 30% at baseline); a segment was considered viable when DOB infusion increased %AC by =< 35% from basal values. Twenty pts showed viable myocardium, Group I (%AC from 21 +/- 7 to 42 +/- 10%); in 22 pts, Group II, %AC was unchanged (24 +/- 7 to 26 +/- 9%). The two groups were comparable for type, site, extension of MI, basal EF, thrombolytic therapy and time of recording from MI. Late potentials were detected in 17 Group I pts and in only one of Group II pts (p < 0.001). In Group I % AC increase during DOB x number of viable segments was positively correlated with fQRS and LAS40 and negatively with RMS40 (r = 0.64 p < 0.005, r = 0.69 p < 0.001 and r = -0.57 p < 0.05, respectively). Thus, early after MI viable myocardial tissue can represent the electrophysiologic substrate for late potentials. SAE parameters are significantly related to both the extension and degree of DOB-induced improvement in contractility of asynergic areas.