In this study, the incidence of acute systolic hypertension (ASH) after acute myocardial infarction and the effects of this complication on the clinical course and prognosis of the disease were studied retrospectively in 950 cases. ASH was characterized as an elevation of systolic blood pressure over 170 mmHg for at least 30 minutes in a previously normotensive subject. There were 50 cases with ASH (5.3%) and 370 (38.9%) with normal blood pressures among the studied 950. The incidences of acute left ventricular failure and of arrhythmias such as sinus tachycardia, atrial and ventricular premature beats and ventricular tachycardia were significantly higher in the group with ASH in comparison with those with normal blood pressures. The group with ASH had also a greater average number of recurrent ventricular fibrillation attacks. The peak SGOT levels were found to be significantly increased in cases in whom the ASH sustained for more than 12 hours. The results suggested that the ASH which occurred predominantly in the early period of acute myocardial infarction was associated with more extensive myocardial damage resulting in a poor clinical course complicated by frequent ventricular arrhythmias and left ventricular failure. It is our conclusion that ASH after myocardial infraction is associated with relatively poor prognosis.