It causes strokes.It can also cause seizures.

Source: http://www.americanheart.org/presenter.jhtml?identifier=2129 High blood pressure is the No. 1 modifiable risk factor for stroke. It also contributes to heart attacks, heart failure, kidney failure and atherosclerosis (fatty buildups in arteries). In some cases, it can cause blindness. The relationship of blood pressure levels to the risk of cardiovascular disease is continuous, consistent and independent of other risk factors. That means the higher your blood pressure, the greater your risk of heart attack, heart failure, stroke and kidney disease.

It overloads your vital organs such as your heart, kidneys and circulatory system. My husband was diagnosed at age 28 and has been on medication to control it. People who don't take their hypertension seriously are looking to cut their life short.

Here is how my doctor explained it. Have you ever left a garden hose turned on in the sun? Have you seen how it will swell and stretch at a weak point and eventually spring a leak somewhere in that swelling? That is what high blood pressure does to your arteries. Your arteries are multi-layered, just like the hose. They have a smooth lining, that presents very little friction to the cells in your blood so nothing sticks to them. They are wound about with tiny muscles that can contract and expand and help keep the artery from swelling. They have a tough outer layer that also helps them keep their shape. When your blood pressure spikes, it puts a lot of pressure on the muscles and outer layer. Eventually it finds a weak spot and either the artery will split and form a leak, or it will swell outward and form an anuerysm. Your blood can't clot while it is moving, so the body patches leaks in arteries with cholesterol. These patches eventually calcify and form plaques, which narrow the artery and slow blood flow. If it slows down to the point where it can clot, you have a heart attack or a stroke. Aneurysms are a different danger. if they swell outward far enough, the blood begins to go around the walls of the aneurysm, leaving a pocket of blood slowly spinning in the middle. That blood is essentially still and can clot. At any time, the moving blood can catch that clot and move it into a narrower part of the artery, closing it off and causing a heart attack or stroke. Aneurysms can also burst, causing you to bleed out, sometimes in seconds. The good news is that exercise and hdl cholesterol helps to "scour" the plaques down and keep them small and smooth so your arteries stay open and blood moves rapidly enough to prevent clots. I don't know if there is anything that helps with aneurysms, besides stenting or blocking them off.