The New York times features an item on research distinguishing between the good, the bad and the indifferent in marriages, and their effect on your health.
Tara Parker-Pope writes (excerpt):
That said [the Brigham Young University study], the conclusion that the health benefits of marriage are dependent on the quality of the relationship has been borne out in other research. For instance, a University of Utah study found that a marital fight that lacked warmth or was controlling in tone could be just as predictive of poor heart health as whether the individual smoked or had high cholesterol. Ohio State University researchers found that wounds heal more slowly when couples have hostile arguments compared with couples who manage conflict without hostility. At the University of Virginia, studies showed that when happy couples held hands, the calming effect on the brain was similar to that caused by pain-relieving drugs. But unhappy couples did not show the same benefit.