To examine the diagnostic significance of precordial ST segment depression in Q wave inferior myocardial infarction, 157 consecutive patients were examined carefully by means of auscultation, ECG, and two-dimensional echocardiography. Precordial ST segment depression was transient (lasting < 72 hours from the onset of myocardial infarction) in 63 patients and persistent (> or = 72 hours) in 40. Twenty-eight patients with persistent, 19 patients with transient, and 14 patients without precordial ST segment depression had advanced asynergy (akinesia or dyskinesia) in the posterior segments, whereas 13 patients with persistent, six with transient, and six without precordial ST segment depression had pericardial rub. Patients with persistent precordial ST segment depression had a significantly higher incidence of severe wall motion abnormality (p < 0.01) and inflammation (p < 0.05) of the posterior wall than the other two groups. In 5 of 40 patients with persistent ST segment depression, pericardial rub was detected in the absence of advanced asynergy in the posterior segments. Although not highly sensitive, persistent precordial ST segment depression appeared to be a fairly specific indicator (specificity 92%) of concomitant posterior involvement with severe wall motion abnormality, inflammation, or both.