To define the incidence and possible significance of new arterial hypertension during myocardial infarction, the records of 87 consecutive infarct patients were examined. Forty-two patients had a history of previous sustained hypertension and were deleted from the study. Of the remaining 45 patients, one was omitted from consideration because of vasopressor therapy for severe hypotension on admission. The study group thus consisted of 44 patients without a history or physical findings of previous hypertension. Nineteen (43%) of these 44 previously normotensive patients demonstrated acute arterial hypertension (blood pressure greater than 140/90 mm Hg) averaging 165/106 mm Hg and persisting for 9 ± 12 hours. The remaining 25 patients had blood pressures averaging 122/76 mm Hg. Infarct location in the 19 acutely hypertensive patients was judged electrocardiographically to be anterior in nine and posterior in ten. Eleven of these 19 patients subsequently had coronary arteriograms. Ten of these 11 demonstrated severe proximal stenoses in the left anterior descending or left circumflex coronary arteries, regardless of location of the infarct. On comparison to our entire series and to the experience of others, we found that new arterial hypertension is more frequent (nearly half our cases) than previously recognized, that it lasts only some hours, and that it does not seem to affect prognosis. The occurrence of such hypertension does not correlate with electrocardiographically defined location of the infarct, but does have a high degree of correlation with severe disease in the proximal left coronary circulation. The possibility that the acute arterial hypertension was mediated by a chemoreceptor near the main left coronary artery is discussed.