A colonoscopy looks inside your large intestine using a long, flexible tube with a tiny camera. Doctors use it to diagnose ulcerative colitis — long-term swelling and sores in the large intestine — and to rule out other conditions with similar symptoms. It also shows how severe the disease is and how much of the large intestine is involved. During the test, doctors take small tissue samples called biopsies to examine closely. People with long-standing ulcerative colitis that affects a significant portion of the colon are at higher risk for colorectal cancer. They need closer, more regular screening. Your care team will decide how often you need a colonoscopy based on your situation.