Having a high level of LDL cholesterol for a long time causes hardening of the arteries ( atherosclerosis Atherosclerosis ). Your arteries clog up with a mixture of cholesterol, fats, and other substances. Your blood has trouble flowing through the clogged vessels. Sometimes blood clots form in the clogged areas and shut off blood flow completely. Shutting off blood flow to your heart causes a heart attack Heart Attack . Shutting off blood flow to your brain causes a stroke Stroke .