We compared the results of 19 mm SJM aortic valve replacements with those of 23 mm SJM aortic valve replacements conducted the same period. [Material & Method] The subjects were 21 patients who underwent valve replacement with a 19 mm SJM prosthesis (the SJM19A group). The age ranged among 16 and 65-year-old, mean of 51. The body surface area was 1.24 to 1.76 m2, mean of 1.48 m2. The post-operative follow-up period was 156 months at maximum and 44 months on average. We compared the SJM19A group with the SJM23A group by chest X-ray and echocardiography. [Results] Two patients in the SJM19A group died soon after surgery. Of the other patient, 19 were categorized NYHA I and one in NYHA II classification in their late phase. Arrhythmia of Lown IVa developed in one patient. The cardio-thoracic ratio decreased from preoperative 60% in the late phase after surgery (p < 0.002) in the SJM19A group, although there was no significant difference with that in the SJM23A group. Echocardiographic improvement in left ventricular hypertrophy was considerable between before and late after surgery in the SJM19A group, while it was not significantly different between the SJM19A and SJM23A group. The mean value of aorta-left ventricle pressure difference in the late stage was 32 mmHg in the SJM19A group and significantly different from the value in the SJM23A group (p < 0.001). This pressure difference tended to be greater as the follow-up period was progressed, while the percentage decrease in the myocardial mass of the left ventricle tended to decreased with longer follow-up period. This data suggests that an increasing level of aorta-left ventricle pressure difference should raise little problem in the mid-term late stage after surgery but possibly cause a serious problem in the long term. Clinical observation should be continued over a long period of time after surgery.