What it likely boils down to is stress -- a well-known contributor to health problems, as well as a potential byproduct of troubled relationships, the scientists said.

In a study of 9,011 British civil servants, most of them married, those with the worst close relationships were 34 percent more likely to have heart attacks or other heart trouble during 12 years of follow-up than those with good relationships. That included partners, close relatives and friends.

The study, in Monday's Archives of Internal Medicine, follows previous research that has linked health problems with being single and having few close relationships. In the new study, researchers focused more on the quality of marriage and other important relationships.

"What we add here is that, 'OK, being married is in general good, but be careful about the kind of person you have married.' The quality of the relationship matters," said lead author Roberto De Vogli, a researcher with University College in London.

De Vogli said his research team is doing tests to see if study participants with bad relationships have any biological evidence of stress that could contribute to heart disease. That includes inflammation and elevated levels of stress hormones.

Another recent study also looked at quality of relationships but had different results. There was no association between marital woes in general and risks for heart disease or early death. But it did find, over a 10-year follow-up, that women who keep silent during marital arguments had an increased risk of dying compared with wives who expressed their feelings during fights. What appeared to matter more for men was just being married; married men were less likely to die during the follow-up than single men.