Most altitude sickness occurs in people who quickly ascend to high altitude. But some people develop altitude-related diseases only after living a long time at high altitude.

Chronic mountain sickness (Monge's disease) is a disease that develops in some people who live at altitudes higher than 10,000 feet (about 3,000 meters) for many months or years. Symptoms include fatigue, shortness of breath, aches and pains, and a blue color to the lips and skin (cyanosis). In affected people, the body overcompensates for the lack of oxygen by overproducing red blood cells. The extra red blood cells make the blood so thick that it may become difficult for the heart to pump enough blood to the organs throughout the body.

Monge's disease is common in the Andes Mountains but has also been seen in high altitude communities in Colorado. In other areas of the world (for example, in Tibet), some people develop a different form of chronic altitude illness marked by high blood pressures in the lungs and decreased function of the right side of the heart, without overproducing red blood cells.