Whether or not stenosis of the proximal segment of the diagonal branch of the heart is life-threatening is determined primarily by the size of the first diagonal branch and the severity of the narrowing of the opening. If the diagonal branch is small or the stenosis is mild, it is basically not life-threatening. If the diagonal branch is large but the degree of stenosis is high, it may be life-threatening. 1. No effect: If the first diagonal branch is very small and the opening is only slightly narrowed, the impact on heart function and blood supply to the myocardium is relatively small in most patients. If there are no obvious clinical symptoms in daily life, life expectancy will not be affected. 2. Impact: If the first diagonal branch is relatively large, the supply area is very extensive, and the degree of stenosis is 75% or 80% or more, it may have an impact on the patient’s life. The decision to stent or perform pharmacologic balloon dilatation is based on the specific location and condition of the lesion. Without early treatment, cardiovascular accidents, such as acute myocardial infarction, may occur and be life-threatening. In case of stenosis of the proximal segment of the diagonal branch of the heart, it is recommended to go to the hospital for prompt and aggressive treatment.