The question whether there is a level of diastolic pressure during treatment below which further reduction of pressure may be harmful rather than beneficial is of great interest. If, as the proponents of this hypothesis maintain, death from CHD among treated hypertensives becomes more rather than less common at very low diastolic pressure, this might explain at least in part why most primary prevention trials of hypertension have failed to show a reduction in CHD mortality. However, as the sceptics have pointed out, the evidence that drug induced lowering of blood pressure is harmful is not of the highest quality, and alternative explanations for excess cardiovascular mortality at low diastolic pressure exist. In the following review of this hotly contested debate it is concluded that both proponents and sceptics may be correct, but that the presence of a J curve should not divert attention from the main benefit of treating hypertension which is a reduction in the risk of fatal and non fatal stroke.