High-density lipoproteins (HDL) are a class of lipoproteins that carry fatty acids and cholesterol from the body's tissues to the liver. About thirty percent of blood cholesterol is carried by HDL.

It is believed that HDL can remove cholesterol within the arteries and transport it back to the liver for excretion or re-use, which is the main reason why HDL-bound cholesterol is sometimes called "good cholesterol", or HDL-C.

A high level of HDL seems to protect against heart disease, whereas low HDL cholesterol levels (less than 40 mg/dL) increase the risk for heart disease.

The National Institute of Health sets HDL levels as follows:

• <40 mg/dL= Low HDL cholesterol, heightened risk for heart disease, <50 mg/dL is the value for women
• 40–59 mg/dL=Medium HDL level
• >60 mg/dL= High HDL level, optimal condition considered protective against heart disease
  

References

Baylor College of Medicine, Lipids Online (January 29, 2001). Heterogeneity of HDL. Retrieved on February 20, 2006.