Ignoring this red flag could have deadly consequences. A new study has found that rectal bleeding is the “strongest predictor of colorectal cancer” in adults under 50 — increasing the likelihood of a diagnosis by an astonishing 850%.

“This research lends support to the question of who does or doesn’t warrant a colonoscopy,” said Dr. Sandra Kavalukas, colorectal surgeon at the University of Louisville School of Medicine and the study’s lead author.

Screening Guidelines May Leave Younger Adults at Risk

Currently, Americans without a family history of colorectal cancer are advised to begin screenings at age 45. But experts caution that this recommendation leaves many younger adults — the group experiencing the sharpest rise in colorectal cancer — dangerously unprotected.

“Many of the early-onset colorectal cancers that I see have no family history,” Kavalukas noted, adding that the study could help physicians determine which younger patients with symptoms might benefit from earlier colonoscopies.

Rectal Bleeding: The Key Symptom Doctors Shouldn’t Ignore

The research analyzed medical records from 443 patients under 50 who underwent colonoscopies at University of Louisville Health between 2021 and 2023. Nearly half were diagnosed with early-onset colorectal cancer, with rectal bleeding standing out as the clearest red flag.

“If they’re 35 and they come in with rectal pain, they probably don’t need a colonoscopy,” said Kavalukas. “But if they come in with a bleeding complaint, they are 8.5 times more likely to have colorectal cancer.”

Family History and Smoking Are Lesser but Notable Factors

Interestingly, 70% of young patients diagnosed had no family history of the disease, and only 13% carried genetic mutations commonly associated with hereditary cancers. While having a family history did double the risk, the presence of rectal bleeding posed a far greater threat.

The study also found that former smokers were nearly twice as likely to develop early-onset colorectal cancer compared to nonsmokers.

When it came to colonoscopies, symptoms were the main motivator: 88% of those diagnosed underwent the procedure due to issues like bleeding, compared with just over half of cancer-free patients.

The findings were presented at the American College of Surgeons Clinical Congress 2025.

A Growing Crisis Among Young Americans

Colorectal cancer rates are climbing among younger adults. According to the Cancer Research Institute, one in five new cases now occurs in people under 55 — a dramatic rise that has been building since the 1990s, even as older adult cases decline.

The causes remain uncertain, but researchers believe modern lifestyles are largely to blame — including sedentary routines, obesity, and diets high in processed foods. Environmental pollutants in air, water, and soil are also suspected contributors.

Because symptoms in younger patients are often dismissed or misdiagnosed, many are diagnosed at later stages — when treatment options are more limited.

Today, colorectal cancer stands as the leading cause of cancer death in American men under 50, and the second deadliest for women in the same age group. In 2023 alone, the disease claimed the lives of 3,750 young adults and affected nearly 20,000 others, according to the American Cancer Society.