Diverticulosis is the presence of uninfected diverticula, or small, bulging, abnormal sacs, in the large intestine. The pouches develop mainly in the sigmoid colon, the last section of the large intestine.

The large intestine absorbs water as it moves fecal matter into the rectum to be excreted from the body. The sigmoid colon, located on the left side of the body, empties into the rectum.

In most cases, diverticula form due to pressure against weak spots that develop within the colon. In the United States and some other Western nations, the sigmoid colon is the most common area in which these sacs develop. By contrast, people in some Asian countries develop sacs in other parts of the colon (which may be due to diet). In very rare cases, the sacs may occur throughout the entire colon.

There may be only a single sac (diverticulum) or there may be hundreds. Most are between 0.1 centimeter (about 0.04 inch) and 1 centimeter (about 0.39 inch) in diameter, although they can exceed 2 centimeters (about 0.79 inch). Larger sacs are often caused by infection. Giant diverticula are rare and can range from 3 centimeters (slightly over an inch) to 15 centimeters (almost 6 inches) in diameter.

Diverticulosis is very common in the United States, especially in older adults. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), about 10 percent of Americans under the age of 40 years have diverticulosis. The condition exists in half of all Americans over the age of 60 years, according to the NIDDK. However, due to the lack of symptoms in most people, the precise incidence is not known.

In most cases, diverticulosis does not cause any complications. However, the presence of diverticula increases the risk of diverticulitis, which occurs when the sacs become infected. According to the NIDDK, between 10 and 25 percent of patients with diverticulosis will develop diverticulitis. Both diverticulosis and diverticulitis together are often referred to as diverticular disease.