49 cases of obstructive cardiomyopathy (37 of the sporadic type, 12 of the familial type), confirmed by phonomechanocardiographic, haemodynamic and/or angiographic investigation and followed up for more than 5 years (mean 7.5 years) without surgical intervention, form the basis of a study on the natural history of this condition. There were 19 deaths (39%), of which 7 were of the familial type (58%) and 12 of the sporadic type (32%). The actuarial survival curves, which were commenced at the time or the first clinical symptom or the date of the first hospitalisation, show that approximately one patient in two dies at or about the age of 40. In 6 cases, death occurred in patients who were greatly improved by medical treatment to the extent of remaining symptomfree up to the time of death. No factors could be found in the initial investigation (phonomechanocardiographic electrocardiographic, radiographic, haemodynamic and angiographic examination (phonomechanocardiographic, electrocardiographic, between the patients still alive and those who had died; each paramater studies had mean values which overlapped in the two groups. Only the age at which the first clinical feature appeared separated the two groups: those patients who died had their first symptom or sign of disordered function at a much younger average age (15.9 years) than the survivors (25 years). This study has confirmed the very serious nature of obstructive cardiomyopathy, especially the familial types, and those with a low age of onset. It emphasises the extreme difficulty in arriving at a prognosis in this condition.