Observation of isolated islands of apparently surviving myocardium within areas of necrotic tissue at the edge of myocardial infarctions has been interpreted by some as histologic evidence of a unique "border zone" region. Serial section reconstruction of transmural canine myocardial infarctions was performed in this study to establish whether these islands were truly isolated or were continuous peninsulas of tissue separated by the plane of section. Three-dimensional analysis of the infarcts revealed no true islands but instead demonstrated a region composed of highly complex interdigitating peninsulas. We conclude that there is an extremely irregular but sharp boundary demarcating normal and infarcted myocardium with no intermediate zone. This observation is discussed in relation to recent data, based on coronary blood flow and creatine phosphokinase analysis, which also demonstrates a sharp boundary between the normal and infarct zones.