Louise Hawkley of the University of Chicago and colleagues also looked at the possibility that depression and stress might account for the blood pressure increase but found that those factors did not fully explain the increase in blood pressure among lonely people age 50 and older.
"Loneliness behaved as though it is a unique health-risk factor in its own right," Hawkley wrote in an article published in the journal Psychology and Aging.
People who have many friends and a social network can feel lonely if they find their relationships unsatisfying, while people who live solitary lives may not be lonely if their few relationships are meaningful and rewarding, Hawkley said.
The study involved 229 people ages 50-68, who were randomly chosen among whites, African-Americans and Latinos who answered questions on loneliness and their connections to others.
The loneliest people saw their blood pressure rise by 14.4 millimeters of mercury more than the blood pressure of their most socially contented counterparts over the four-year study period.
"Loneliness is characterized by a motivational impulse to connect with others but also a fear of negative evaluation, rejection and disappointment," Hawkley said.