Cases of breast cancer in women under 40 are rising across Europe, research suggests.
Experts say it is unclear whether this is due to improved diagnosis or new risk factors.
A study in Cancer Epidemiology found cases rose by about 1% a year between 1990 and 2008 in seven countries.
Breast cancer is the most common cause of cancer among women globally, and the leading cause of cancer death.
Cancer in young women is rare - only about 5% of all breast cancers are in women under the age of 40.
Despite this, it is the leading cause of death in young adult women.
Researchers in France and Italy studied trends in breast cancer in women under 40 in Belgium, Bulgaria, France, Italy, Portugal, Spain and Switzerland over an 18-year period.
They found that on average cases rose by about 1% a year in women under 40, with the greatest rise in women under 35.
It is unclear if this is due to a rise in risk factors or improving methods of diagnosis, they say.
"The rise in incidence was greater for women under 35 and for ductal carcinomas [a type of tumour in the ducts of the mammary gland]," the researchers, led by Dr Brice Leclere, of the GRELL working group, write in the journal Cancer Epidemiology.
"This increase can be due to a rise in risk factors and/or changes in diagnosis and surveillance practices, but we could not clearly distinguish between these two non-exclusive explanations."
Commenting on the research, Jessica Kirby, Cancer Research UK's health information manager, said: "It's worrying to see a rise in breast cancer rates in younger women in Europe, but this study didn't include the UK.
"Rises in breast cancer rates could be caused by a range of things that can increase the risk of breast cancer, such as women having fewer children and having them later in life, or greater awareness and diagnosis in this group.
"Women can reduce the risk of breast cancer by keeping active and cutting down on alcohol. Also get to know your breasts and, if you notice any change, tell your doctor without delay."