Thirty patients who died in the acute phase of an inaugural posterior myocardial infarction were classified in two groups: group A, 14 cases, and group B, 16 cases without extension to the right ventricle. The autopsy examination showed the following differences. In group A: 1. The right coronary artery (RCA) gave off the posterior descending artery in all cases; 2. The RCA had diffuse atheromatous change in 10 cases (p less than 0,05); 3. Total occlusion of the RCA by thrombosis was constant (p less than 0,01) and was always proximal to or near the origin of the right marginal artery (p less than 0,001); 4. Postero-septal extension was constant and nearly always transmural (p less than 0,001).