More young people under 50 are being diagnosed with bowel cancer, two studies of the disease in European and high-income countries have found.
Although total numbers of cases in young people remain low, the studies highlighted a sharp rise in rates in 20 to 29-year-olds.
Researchers are not clear why this is happening, but say obesity and poor diet could be factors.
Experts urged doctors not to ignore symptoms in young people.
In most of Europe, bowel cancer screening programmes start at the age of 50 because cases of the disease are much higher among this older age group.
As a result, countries with established programmes, like the UK, have seen bowel cancer rates in the over-50s fall.
But recent research suggests rates are now rising more steeply among under-50s - and there have been calls for screening to start at 45 instead, in the US particularly.
Sharp rise in young
In a study in the journal Gut, Dutch researchers analysed trends in 20 European countries, including the UK, Germany, Sweden and France, using data from more than 143 million people.
They found a rise in cases of bowel cancer between 1990 and 2016 in most countries - with the most significant increase among people in their 20s.
For them, bowel cancer incidence increased from 0.8 to 2.3 cases per 100,000 people over 26 years - with the sharpest rise in rates, of 7.9% per year, occurring between 2004 and 2016.
But there was no increase in deaths from bowel cancer for this age group, the study found.
Among people in their 30s, rates of bowel cancer also rose - but less steeply - and among 40-somethings rates fluctuated.
The researchers, from the Erasmus MC University Medical Center in Rotterdam, said if the trend continued, screening guidelines may need to be reconsidered.
Another study, in the Lancet Gastroenterology & Hepatology, appeared to confirm the trend among young adults in high-income countries, including the UK, Australia, Canada and New Zealand.
It found a 1.8% increase in colon cancer cases and 1.4% rise in rectal cancer cases in people under 50 in the UK between 1995 and 2014 .
Over the same period, there were decreases in bowel cancer cases of 1.2% in the over-50s.
The findings were similar in many of the countries studied.