Lonely people are more prone to developing high blood pressure in later life, say researchers.
They found chronic feelings of loneliness push up blood pressure over time, causing a marked increase after four years in people aged over 50.
A new study is the first to show a direct link between loneliness and high blood pressure, known as hypertension, which raises the risk of heart attack and stroke.
U.S. researchers considered whether depression and stress were pushing up blood pressure but found they were only partly responsible.
Dr Louise Hawkley, of the research team from Chicago University, said 'Loneliness behaved as though it is a unique health-risk factor in its own right.'
She said living alone did not necessarily mean people were lonely - some people appeared to have busy lives and a good social network but still felt lonely, which puts them at risk.
Previous research has suggested that individuals who feel alone are twice as likely to develop Alzheimer's disease as those who experienced little loneliness.
The latest research involved 229 people aged 50 to 68 who were part of a long-term study on ageing.
Members of the group were asked a series of questions to determine if they perceived themselves as lonely.
They were asked to rate connections with others through a series of topics, such as 'I have a lot in common with the people around me,' 'My social relationships are superficial' and 'I can find companionship when I want it."
During the five-year study, Dr Hawkley found a clear connection between feelings of loneliness reported at the beginning of the study and rising blood pressure over that period.
She said 'The increase associated with loneliness wasn't observable until two years into the study, but then continued to increase until four years later.'
Even people with modest levels of loneliness were affected, says a report in the journal Psychology and Aging (sic).
Among all the people in the sample, the loneliest people saw their blood pressure go up by 14.4 mm more than the blood pressure of their most socially contented counterparts over the four-year study period.
Hypertension affects 16 million Britons and is a major risk factor for heart disease, stroke, and kidney failure.
A high blood pressure reading is one that exceeds 140/90 millimetres of mercury.
The first figure, the systolic pressure, corresponds to the 'surge' that occurs with each heart beat.
The second, diastolic, reading is the pressure during the 'resting' stage between beats.
Dr Hawkley, senior research scientist with the Center for Cognitive and Social Neuroscience, said people who have many friends and a social network can feel lonely if they find their relationships unsatisfying.
Conversely, people who live rather solitary lives may not be lonely if their few relationships are meaningful and rewarding.
She said for some people the emotional costs of making and keeping friends appear to outweigh the benefits, which may contribute the blood pressure increase.
She said 'Loneliness is characterized by a motivational impulse to connect with others but also a fear of negative evaluation, rejection and disappointment.
'We hypothesise that threats to one's sense of safety and security with others are toxic components of loneliness, and that hyper-vigilance for social threat may contribute to alterations in physiological functioning, including elevated blood pressure' she added.