A slew of new research during the past few years shows that marital stress can play a significant role in a person's overall health — increasing risk for everything from chronic pain to a heart attack. A low-stress marriage can even increase survival chances when a health problem strikes.

While it's long been known that people who are married tend to be healthier and live longer than unmarried people, scientists are increasingly turning their attention to whether the quality of the marriage matters. Some of the resulting studies have shown that the risk of a bad marriage is as strong as other medical risks. Among patients who suffered congestive heart failure, those with good marriages were more likely to survive. One study linked marital distress to dangerous thickening of the heart wall, just like smoking.

And while we've long known that stress is a major risk factor for many health problems, marital stress appears to be a bigger hazard than other types of stress simply because it's so personal.

"You can't escape marital stress the way you can other types of stress," says Annmarie Cano, assistant psychology professor at Wayne State University in Detroit and author of a January study on the links between chronic pain and marriage stress. "Most people think of marriage as a comfort zone and a place where you can relax, but when that is stressed, there is no safe haven."

The problem is, many people aren't aware how much their marriage is affecting stress levels. Studies have shown that arguments in couples who have been married for decades can increase stress hormones that weaken the immune system. Research has linked stress hormones with a number of health problems, making a person more susceptible to illness, slowing wound-healing and even interfering with the effectiveness of a vaccine.

The most surprising research has focused on a group of newlyweds, who, by all accounts, seemed happy, even "blissful" in their relationships. But Ohio State University researchers asked the 90 couples to answer questions about their marriage, videotaped them discussing a stressful topic and took blood samples to measure hormones known to inhibit or enhance the immune system. The couples who appeared to become the most agitated and hostile in the videotapes were more likely to see increases in hormones that weaken the immune system. Levels of an immune-boosting hormone also dropped.

Years later, researchers found the couples who eventually divorced had shown significant elevation in three of four immune-weakening hormones. Because those changes were detected in newlyweds, the research shows that not only did the hormones predict divorce risk, but the study also showed that marital stress, long before it's obvious, can have a measurable impact on immune-system health.

The same researchers are now studying the role of marital stress on wound healing. The researchers are inflicting small pea-size blisters on the arms of each spouse, studying whether positive interaction with each other can lead to faster healing by lowering the stress hormone cortisol. Stress hormones can slow the delivery of compounds that start the healing process.

"Marriage stress is unique because it basically takes what should be your primary source of support and makes it your primary stress," says professor Janice Kiecolt-Glaser of Ohio State.

This month, the Harvard Men's Health Watch newsletter examined the relationship between marital stress and heart health, highlighting a study of 72 patients who answered questions on the Dyadic Adjustment Scale.

The study showed that marital distress was linked with a thickening of the left ventricle of the heart, as seen on an echocardiogram, just like smoking and excessive drinking. But job stress didn't have the same effect.

How much you interact with your spouse in a good or bad marriage can also influence your health. The same study found that among people in unhappy marriages, those who spent less time with a spouse had lower blood pressure than those who had lots of contact. Among those in good marriages, people who spent a lot of time with their spouse had even lower blood pressure.

"You can measure the physiological effect of a stressful interpersonal relationship," says Harvard professor Harvey B. Simon, editor of the newsletter.

Dr. Simon, an internist, says he spends a lot of time talking to patients about their personal lives and stress levels. He urges them to exercise, meditate, pursue hobbies and other activities that make them happy and seek personal or marriage counseling.

But while it's clear that a bad marriage can drastically increase stress, it's not yet known whether it's better, in terms of overall health, to try to improve a troubled relationship or to get a divorce — which itself is an extremely stressful life event.

Even in good marriages, the way a couple interacts appears to affect health. A Yale study asked 305 couples married an average of 43 years to name their confidante or greatest source of emotional support. Surprisingly, a couple in which a woman with children named her husband but the husband didn't name her was significantly more likely to be alive after six years than other couples, says Roni Beth Tower, now adjunct assistant professor at Teachers College, Columbia University. One reason may be that being needed, by either your children or your wife, is better for health than having someone to lean on.

To take part in an Internet study of love and longevity, go to www.tc.columbia.edu/research/longevity.