Added to things like heredity and diet, loneliness might contribute to high blood pressure in people over 50.
Researchers say the loneliest people they studied had blood pressure readings up to 30 points higher than those who are surrounded by friends or family.
And that suggests loneliness might be just as bad as being overweight or inactive.
Scientists say the results are "stunning" especially since earlier studies suggest more than nine million Americans often feel isolated or left out.
University of Chicago researcher Louise Hawkley says there's also a big hint for treatment in their findings.
She says blood pressure could improve if a patient just takes steps to get involved like "do volunteer work."