We studied the influence of exercise level, severity of coronary artery disease (CAD), presence of previous myocardial infarction (MI), anterior or diaphragmatic, on the clinical value of exertional Q wave changes (Delta-Q). We retrospectively evaluated the exercise electrocardiograms of 62 patients without angiographic evidence of CAD and 133 patients with CAD; 28 of them had single (SVD) and 105 multivessel disease (MVD). Forty-one patients had a previous diaphragmatic MI and 23 anterior. The sensitivity, specificity and predictive value of Delta-Q were compared to the ST criterion. The exercise level affected Delta-Q. ST and Delta-Q had similar specificity and predictive values. The extent of CAD did not affect the sensitivity of Delta-Q and this method was better than ST to detect SVD patients. The Delta-Q criterion was equally as efficient as ST in MVD patients without MI and with diaphragmatic MI. The loss of septal forces on resting electrocardiograms made useless Delta-Q analysis on patients with anterior MI. The improvement of sensitivity in SVD patients by Delta-Q might be of clinical value since these latter are frequently not diagnosed by the ST criterion.