Denver, the capital of the American state of Colorado, has the nickname the Mile High City because, at one of its points, it measures 5,280 feet (1,609 m) in height. This measurement is exactly one mile above sea level. A round brass piece embedded in one of the steps of the Colorado State Capitol building's Western entrance marks the exact spot in which Denver's altitude is precisely one mile high.

Unlike most American cities that were centered close to a railroad, seaport or other hub of transportation, Denver was an early gold center. Denver was founded in 1858 during the Pikes Peak Gold Rush and incorporated in 1861. Being so far up from sea level, the Mile High City offers beautiful views of the Rocky Mountains. Although Denver is Colorado's largest city, many people also appreciate the outdoor activities its natural beauty offers.

As many travelers aren't used to such elevated altitudes as the Mile High City has, it may take some people a few days to adjust. For some, no adjustment is needed while others get altitude sickness. Altitude sickness often includes headaches and a queasy stomach. Drinking a lot of water can help ease the symptoms. Drinking enough water is also necessary in Denver as well as most of Colorado, as the climate tends to be dry.

People prone to altitude sickness should avoid alcohol and strenuous activity for at least a couple of days. Those who live at or close to sea level may be especially prone to altitude sickness upon arriving in the Mile High City. A doctor may prescribe medication such as acetazolamide to help prevent or reduce the headaches and queasiness associated with altitude sickness.

In cases of more severe forms of heart or lung disease as well as high blood pressure, patients should check with their doctor before traveling to Denver or other places with increased altitude. Proper oxygen levels in higher elevations such as the Mile High City can be an issue for some people with health problems; becoming short of breath is common. Those who are healthy, fit and younger are generally less prone to altitude sickness.

Although Denver, Colorado, is famous for being the Mile High City, there are other cities in the world with even higher altitudes. Potosi, Bolivia, in South America, is more than two miles high. Other cities more than a mile high are Lhasa, Tibet, and Lijiang, China.