Causes and Diagnoses

Causes and Diagnoses of Colon and Rectal Cancer

The exact cause of most colon and rectal cancer is unknown. But, certain risk factors could be contributing causes, including:

  • Age: Colon and rectal cancer can affect people at any age, although most are over 50.
  • Race and ethnicity: African-Americans have the highest risk for colorectal cancer of all racial groups in the United States. Jews of Eastern European descent (Ashkenazi Jews) have the highest colorectal cancer risk of any ethnic group in the world.
  • Diet: Colorectal cancer is often associated with a diet high in red and processed meats.
  • Personal history of colorectal polyps: Benign growths on the wall of the colon or rectum are common in people age 50 and older, and may lead to colorectal cancer.
  • Personal history of colorectal cancer: People who have had colorectal cancer are at higher risk for having it again.
  • Family history: People with a strong family history of colorectal cancer or polyps in a first-degree relative (a parent or sibling before the age of 45 or two first-degree relatives of any age) have an increased risk for colorectal cancer.
  • Ulcerative colitis or Crohn's disease: People who have an inflamed colon lining have a higher risk of colorectal cancer.
  • Inherited conditions: These include familial adenomatous polyposis or hereditary nonpolyposis colon cancer, also known as Lynch syndrome.
  • Obesity: Men and women with a body mass index (BMI) over 30.
  • Physical inactivity: Adult men and women should get two-and-a-half hours weekly of aerobic activity such as walking.
  • Heavy alcohol consumption: Men who drink more than five drinks a day and women who drink more than four drinks a day have a higher risk of colorectal cancer.
  • Type 2 diabetes: Colorectal cancer and type 2 diabetes share some of the same risk factors.
  • Smoking: Smokers have a higher risk of many cancers, including colon and rectal cancer.

Talk to your physician about lowering your risk factors for colon and rectal cancer.

A small percentage of colon and rectal cancers may be caused by inherited gene mutations. If you have a family history of colorectal cancer, you may want to consider genetic testing. The American Cancer Society recommends that anyone receiving genetic tests consult a physician or geneticist, who is qualified to explain the significance of these test results.

Lynch syndrome testing may be recommended for patients with a family history of colorectal cancers.


Colon and rectal cancer is the third most common cancer in both men and women, and the second leading cause of cancer-related deaths in the United States. Fortunately, 90 percent of colorectal cancer cases are curable if diagnosed early. Regular colonoscopy screenings are recommended for anyone age 50 years and older.

Colorectal cancer screening benefits people without symptoms or strong risk factors. Screening procedures include:

  • Colonoscopy: A physician views the entire length of the colon with a flexible, lighted tube.
  • Sigmoidoscopy: A physician views a portion of the large intestine with a flexible, lighted tube.
  • Biopsy: A physician takes a small sample of tissue during a colonoscopy or sigmoidoscopy to examine under a microscope.
  • Digital rectal exam (DRE): A physician uses a finger to check for abnormalities of the colon and prostate.
  • Fecal occult blood test: This test looks for microscopic or invisible blood in the stool.
  • Barium enema: This X-ray procedure uses barium (a metallic, chemical, chalky liquid) to coat the inside of organs so that they will appear on X-ray images.


Diagnostic testing includes screenings in addition to other procedures, including:

  • Blood count: This tests for anemia, which can result from a bleeding tumor.
  • Imaging tests: Procedures such as CT scan, PET scan, ultrasound or MRI of the abdomen may be done to look for tumors or other problems. These tests may also be done if colon or rectal cancer has already been diagnosed to help determine the extent (stage) of the cancer.

Related Resources

Sigmoidoscopy Brochure: Learn more about the preparations and care needed before and after this procedure. English | Spanish