Barlow's syndrome has become a regular, often-used and very often misused diagnosis. Its description followed extensive, prolonged and detailed clinical observation by JB Barlow and his co-workers. This major research effort was necessary because of the protean manifestations of the condition. The differentiation of Barlow's syndrome from other conditions with similar and sometimes identical symptoms requires clear and unambiguous criteria. These criteria were identified by penetrative clinical research. Consequently, it became possible to diagnose Barlow's syndrome with a high degree of specificity. Almost equally important were the gains made in understanding various conditions with similar symptoms but totally different management. An example of which, understanding some of the electrocardiographic patterns that emerge on effort in patients with ischaemic heart disease. Similarly, understanding mitral valve billow led to a greater knowledge of the entire pathophysiology of the mitral valve closure and important aspects of mitral regurgitation. Primary mitral valve billow, Barlow's syndrome, resulted from clinical research of the highest quality and has had a major application in clinical medicine.