In a retrospective study, 29 patients at least 20 years of age with known aortic stenosis are reported who had the peak systolic gradient (PSG) measured on at least two occasions without an intervening surgical procedure or episode of endocarditis. In these 29 patients, there were 31 intervals available for evaluation with a mean follow-up time of 43.5 months (1 week to 120 months). In 16 of the 31 intervals, the PSG increased by 50% or more and in 15, it did not. In the group where the PSG increased, the average rate of increase was 1.3 mm. Hg/month with the most rapid gradient increase at 3.8 mm. Hg/month. Progression to high gradient was correlated with the development of angina pectoris or left ventricular hypertrophy by voltage and ST-T wave changes. In this study, other symptoms were not helpful in predicting an increase in severity. It is still recommended, however, that any patient with aortic stenosis and the development of symptoms of congestive heart failure or exertional syncope should be suspected of having progressed to severe aortic stenosis and should be restudied.