There is hopeless confusion in the English-speaking world about the determination of the diastolic blood pressure by sphygmo manometry. In Britain generations of students have been taught that the point of muffling of the Korotkow sounds (often referred to as the fourth phase) should be taken as the index of diastolic pressure, and until recently all the major British studies on hypertension have been based on this practice. Workers in the United States, however, have long favoured the point of disappearance of the Korotkow sounds (the fifth phase) as the index of diastolic pressure. This disagreement was evident in the joint report of the Cardiac Society of Great Britain and Ireland and the American Heart Association (AHA)' published in 1939 and became explicit in the recommendations of the Scientific Council of the AHA2 in 1951. Many important American studies on the epidemiology and treatment of hypertension used fifth phase recordings-notably the Build and Blood Pressure Study of the Society of Actuaries of Chicago3 and the Veterans Administration Co-operative study on antihypertensive agents.4 The Framingham study,5 on the other hand, used fourth phase recording. In 1967 agreement seemed to have been reached when a subcommittee of the Postgraduate Education Committee of the AHA concluded that the fourth phase should be regarded as the best index of diastolic pressure. But in 1972, or possibly earlier, the Hypertension Detection and Follow-up Programme Collaborative Group in the USA decided to use the fifth phase,7 and it has recently been intimated8 that the fifth phase is to be used in the British Medical Research Council's mild hypertension treatment trial. So now the confusion is worse than ever. Two questions arise: Firstly, Is the issue important ? Secondly, What can be done about it ?