Nervy people may be more at risk of aggressive cancer, new research suggests.
Scientists found that anxious mice were worse affected by skin tumours than less fearful animals.
Stressed and worried humans could be susceptible in the same way, they believe.
Researchers in the US first identified laboratory mice with nervous personalities that avoided the dark or open spaces.
The hairless animals were then exposed to ultra violet radiation at levels equivalent to those experienced by humans who spend too long in the sun.
After a few months, the mice developed skin tumours -- as did a group of non-anxious mice.
The difference was that the nervy mice grew more tumours, and only they went on to develop invasive skin cancer.
The research appears in the online journal Public Library of Science.
Lead author Dr Firdaus Dhabhar, from Stanford University Medical Centre in California, said: "It's bad enough that cancer diagnosis and treatment generates stress and anxiety, but this study shows that anxiety and stress can accelerate cancer progression, thus perpetuating a vicious cycle.
"The goal is to ameliorate or eliminate the effects of anxiety and chronic stress, at least at the time of cancer diagnosis and during treatment."
Chronic stress has already been linked to cancer and other problems. But the latest study is said to be the first to show a biological connection between having an anxious disposition and greater threat from cancer.