Forty-two patients with pure mitral stenosis underwent a prospective, longitudinal study with two or more cardiac catheterizations 1 to 10 years apart and without intervening surgical treatment. The majority of patients had not previously had mitral valve surgery; in 16, the period of observation followed valvotomy. Twenty-seven patients showed evidence of progressive obstruction of the mitral valve; 15 showed no change in the calculated mitral valve area. In the group of patients with progressive disease (stenosis or restenosis) there was a consistent clinical and hemodynamic deterioration: 25 of the 27 showed increasing disability. In the nonprogressive group, seven of the 15 showed increasing symptomatology, most often related to the development of atrial fibrillation and a fall in cardiac output. Possible causes for the different course of mitral stenosis in the two groups are reviewed. The probability that trauma due to altered flow patterns through a stenotic valve produces progressive stenosis is suggested.