Harmful Effects of Smoking
Smoking harms nearly every organ of the body; causing many diseases and reducing the health of smokers in general. The adverse health effects from cigarette smoking account for an estimated 438,000 deaths, or nearly 1 of every 5 deaths, each year in the United States.
More deaths are caused each year by tobacco use than by all deaths from human immunodeficiency virus (HIV), illegal drug use, alcohol use, motor vehicle injuries, suicides, and murders combined.

Cancer
Cancer is the second leading cause of death and was among the first diseases casually linked to smoking.

Smoking causes about 90% of lung cancer deaths in women and almost 80% of lung cancer deaths in men. The risk of dying from lung cancer is more than 23 times higher among men who smoke cigarettes, and about 13 times higher among women who smoke cigarettes compared with never smokers.
Smoking causes cancers of the bladder, oral cavity, pharynx, larynx (voice box), esophagus, cervix, kidney, lung, pancreas, and stomach, and causes acute myeloid leukemia.

Rates of cancers related to cigarette smoking vary widely among members of racial/ethnic groups, but are generally highest in African-American men.

Cardiovascular Disease (Heart and Circulatory System)
Smoking causes coronary heart disease, the leading cause of death in the United States. Cigarette smokers are 2?4 times more likely to develop coronary heart disease than nonsmokers.

Cigarette smoking approximately doubles a person's risk for stroke.

Cigarette smoking causes reduced circulation by narrowing the blood vessels (arteries). Smokers are more than 10 times as likely as nonsmokers to develop peripheral vascular disease.

Smoking causes abdominal aortic aneurysm.

Respiratory Disease and Other Effects
Cigarette smoking is associated with a tenfold increase in the risk of dying from chronic obstructive lung disease.  About 90% of all deaths from chronic obstructive lung diseases are attributable to cigarette smoking.
Cigarette smoking has many adverse reproductive and early childhood effects, including an increased risk for infertility, preterm delivery, stillbirth, low birth weight, and sudden infant death syndrome (SIDS).
Postmenopausal women who smoke have lower bone density than women who never smoked. Women who smoke have an increased risk for hip fracture than never smokers.

Benefits of Cessation
People who stop smoking greatly reduce their risk of dying prematurely.
Benefits are greater for people who stop at earlier ages, but cessation is beneficial at all ages.

Smoking cessation lowers the risk for lung and other types of cancer. The risk for developing cancer declines with the number of years of smoking cessation.
Risk for coronary heart disease, stroke, and peripheral vascular disease is reduced after smoking cessation. Coronary heart disease risk is substantially reduced within 1 to 2 years of cessation.

Cessation reduces respiratory symptoms, such as coughing, wheezing, and shortness of breath. The rate of decline in lung function is slower among persons who quit smoking.
Women who stop smoking before or during pregnancy reduce their risk for adverse reproductive outcomes such as infertility or having a low-birth-weight baby.

For research references and more information, please see the Centers for Disease Control.