For more than 20 years, social epidemiologist Jeff Levin, Ph.D., M.P.H., has been collecting data to see if there's a link between faith and health.

"About 80% to 90% of these studies show there is something positive going on. We're swimming in empirical evidence."

Among those documenting the case is Harold G. Koenig, M.D., professor of psychiatry at the Duke University Medical Center: "Our studies have shown those who benefit most are those who both attend religious services and practice personal belief at home such as reading religious literature and prayer," he says.

According to his research as director of the Center for Spirituality, Theology and Health, Dr. Koenig, who also has led seminars on the subject for the Harvard Medical School Continuing Medical Education program, has found:

• People who regularly attend church service, pray individually, and read the Bible are 40% less likely to have diastolic high blood pressure than those who seldom participate in these religious activities.
• People who attend religious services regularly may have stronger immune systems than their less-religious counterparts.
• People who attend church regularly are hospitalized less often and leave the hospital sooner than people who never or rarely participate in religious services.
• The deeper a person's religious faith, the less likely he or she is to suffer depression during and after hospitalization for physical illness.
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• Religious people tend to have healthier lifestyles. People who attend church at least weekly have about one-third the rate of alcohol abuse and are about one-third as likely to smoke as those who seldom participate in congregational worship.
• Religious youth show significantly lower levels of drug and alcohol abuse, premature sexual involvement, and criminal delinquency than their non-religious peers. They also are less likely to express suicidal thoughts or make actual attempts on their lives.

Levin, who has received grants from many sources, including the National Institutes of Health, found that faith is an especially potent source of well-being.

"The big challenge for us is to answer the why question, what does all of this mean?"

In his book "God, Faith and Health," Levin examined more than 200 studies on faith and health. Among the common links are:

• On the average, people who are more religious tend to have healthier lifestyles.
• Attending church, mosque or synagogue provides a social structure. "Being around other people you receive support. We know that social support is good for people's health," he says.
• The practice of prayer, meditation or public worship services elicits positive emotions. A variety of new scientific research suggests that our emotions can lead to psychological changes that benefit our health.
• New research on optimism and hope is suggesting a possible placebo effect that promotes healing.

The studies Levin looked at involved a range of ages, ethnic backgrounds and religious affiliations. They involved a variety of research methods, including representative samples, longitudinal and psychiatric studies, as well as analysis by condition, such as lung disease or cancer.

Levin and Dr. Koenig are part of a growing movement to quantify the impact of religion on health.

Others are working on the subject too. For instance, a Columbia University research report concluded that people with religious faith are markedly less likely to abuse alcohol and illegal drugs than non-believers.

Harvard University's Pluralism Project has joined with Boston Medical Center's Healing Landscape project to explore the extent of religious healing in an American city. The project is producing a data bank of information about the diversity of religious healing practices in urban America.

While research supports a connection between faith and physical well-being, the evidence is stronger for the benefits of faith on emotional and mental health, says Dr. Koenig, author of many books on the subject.

For most people, faith's power involves a healing of the intangible spirit, and of relationships with others, he adds.

In fact, illness tends to bring spirituality to the surface.

"As people become sicker and struggle with more suffering they can become very deeply spiritual," Dr. Koenig says. "They may look sick or be struggling with depression, but they may be deeply spiritual and have a very strong faith connection."

This is because without faith, illness and suffering is devoid of meaning for patients and their loved ones, Dr. Koenig says. "It gives these difficult conditions a sense of purpose, that somehow a good thing can result. That God can transform this horrible situation into something good, or it can have a benefit to those around them."

The research suggests that intensity of belief and practice is an affecting characteristic in the relationship between health and spiritual practice. In one study patients were asked to rank the importance of their faith, they were given a range of possible answers from not important to strongly important.

"There is some evidence from some of these types of questions, that people who answer in the stronger category tend to do better in health and in psychological well being," Levin says.

Yet the research does not suggest that non-religious people can't also be healthy.

"There are unquestionably people who do not practice religion and are perfectly healthy and get along just fine. And there are people who are very religious and suffer illnesses," Levin says.

But with research unquestionably documenting a health benefit for members of religious groups, Levin concludes: "Spirituality in the broadest sense can be a vitally powerful resource in the lives of many human beings for those who participate in spiritual activities on a regular basis."