Fifty of 305 patients studied angiographically had segmental early relaxation phenomenon (SERP) of the anterolateral or apical left ventricular wall. Fourteen of the 50 patients had cardiac abnormality other than, or in addition to, coronary occlusive disease. Of the remaining 36 patients, 35 had significant lesions in the left anterior descending artery (LAD) and one had occlusion of the proximal left circumflex artery. SERP was noted in 35 (37.2 per cent) of 94 patients with LAD disease who did not have severe anteroapical contraction abnormality. SERP was found in none of 49 patients who had neither significant coronary occlusive disease nor other cardiac abnormality. Eight patients were restudied after bypass grafting of the LAD. Three patients with patent grafts no longer demonstrated SERP. Five had persistent SERP, but four of these had occluded LAD grafts or nongrafted disease in other vessels supplying the anterolateral wall. Myocardial ischemia appears to be one cause, but probably not the only cause, of SERP.