A report published Tuesday by the Centers for Disease Control and Prevention said just 65.1 percent of adults 50 to 75 get the recommended screenings.
While screening rates increased from 54 percent to 65 percent between 2002 and 2010, the numbers from last year appear to show a leveling off.
It's a "really disturbing fact," said CDC director Dr. Tom Frieden Tuesday.
Some 102,500 people will be diagnosed with with colon cancer this year, and another 40,000 will be diagnosed with rectal cancer, the American Cancer Society estimates. Colonoscopies -- and more rarely sigmoidoscopies -- examine for polyps, which can turn to cancer, in the colon or the rectum. Catching the polyps early can prevent them from developing into cancer.
The CDC said doctors may be to blame for more people not getting tested. Two in three adults who have never been screened have a regular doctor and health insurance.
Frieden did say he hopes the Affordable Care Act's provisions for preventative care will encourage more people to get tested.
"Through ACA, more Americans will have access to preventative colon cancer screening at no cost to patients," he said.