Digitised M-mode echocardiography was used to study the changes in left ventricular diastolic function over a 3-year period in 11 patients with hypertrophic cardiomyopathy an 14 normals. Compared to normal, in hypertrophic cardiomyopathy, isovolumic relaxation was prolonged (P less than 0.001) and mitral valve opening delayed relative to minimum dimension (P less than 0.001). There was a wide range of values for the peak rates of dimension increase and wall thinning, and although the means were normal, 6 and 8 patients respectively were outside the normal range. There were no significant mean changes in function during the 3.4 +/- 0.3 years of follow-up, but, in 3 patients, marked alterations in relaxation were observed. They showed a gross reduction in the delay in mitral valve opening (125 to 55 125 to 35 and 110 to 75 msec). There was little overall change in isovolumic relaxation in two, but in one patient it reduced from 95 to 50 msec. In most patients with hypertrophic cardiomyopathy, relaxation and diastolic function appear to remain stable over a period of 3 years, and none had an apparent deterioration. Some patients may have an apparently spontaneous "improvement" in function similar in extent to that described due to the therapeutic action of calcium antagonists.