A report in the New England Journal of Medicine said that people recently told they had cancer were more likely to have heart attacks or commit suicide.
It's based on a Swedish study of more than six million adults over 15 years.
Experts suspect this tough diagnosis may trigger post traumatic stress disorder and depression, increasing the risk of suicide.
It could lead to a rise in heart rate and blood pressure, which raises the increase in heart attacks and stroke.
The study put doctors on alert to watch for signs of stress in their patients.