We studied by serial cardiac catheterisation eight patients with the dilated form of primary endocardial fibroelastosis in whom congestive heart failure disappeared with treatment. All remained without symptoms for at least three years before recatheterization. Four patients showed regression of the abnormal electrocardiographic findings, three showed persistence, and one showed progression of electrocardiographic left ventricular overload pattern. On first cardiac catheterisation all patients had a dilated left ventricle with a mean ejection fraction of 0.36. In six of the patients repeat cardiac catheterisation showed left ventricular dilatation with a diminished ejection fraction (mean 0.32). Left ventricular end-diastole pressure was raised (12 to 28 mmHg, mean 19 mmHg). In this group were included the three patients with persistence and one with progression of the abnormal electrocardiographic findings, and two of the four patients with regression of these findings. The highest left ventricular end-diastolic pressure was found in a patient in whom the abnormal electrocardiographic findings almost reverted to normal. In the two remaining patients with reversion of the electrocardiographic abnormalities repeat cardiac catheterisation showed nothing abnormal. Our findings indicate that "cure" in primary endocardial fibroelastosis is incomplete. These findings may be the cause of sudden death or late clinical deterioration in some reported patients with "cured" primary endocardial fibroelastosis. The electrocardiogram is of little value in assessing these processes.