Avocados contain oleic acid, a monounsaturated fat that may help to lower cholesterol. In one study of people with moderately high cholesterol levels, individuals who ate a diet high in avocados showed clear health improvements. After seven days on the diet that included avocados, they had significant decreases in total cholesterol and LDL cholesterol, along with an 11% increase in health promoting HDL cholesterol.

Avocados are a good source of potassium, a mineral that helps regulate blood pressure. Adequate intake of potassium can help to guard against circulatory diseases, like high blood pressure, heart disease or stroke. In fact, the U.S. Food and Drug Association has authorized a health claim that states: "Diets containing foods that are good sources of potassium and low in sodium may reduce the risk of high blood pressure and stroke."

 
One cup of avocado has 23% of the Daily Value for folate, a nutrient important for heart health. To determine the relationship between folate intake and heart disease, researchers followed over 80,000 women for 14 years using dietary questionnaires. They found that women who had higher intakes of dietary folate had a 55% lower risk of having heart attacks or fatal heart disease. Another study showed that individuals who consume folate-rich diets have a much lower risk of cardiovascular disease or stroke than those who do not consume as much of this vital nutrient.
 
A link posting all around information on avocados can be found here, on a wonderful web site called Worlds Healthiest Foods.
 
Additional avocado sites include: