Most doctors agree that breastfeeding is best for babies’ health. Now a large study suggests that the practice benefits mothers as well: women who have breast-fed, it says, are at lower risk than mothers who have not for developing high blood pressure, diabetes and cardiovascular disease decades later, when they are in menopause.

The benefits increase with duration of past breast-feeding, the study found. Women who had breast-fed for more than a year in their entire lifetimes were almost 10 percent less likely than those who had never breast-fed to have had a heart attack or a stroke in their postmenopausal years. They were also less likely to have diabetes, hypertension and high cholesterol.

The study found that even those postmenopausal women who had breast-fed for just one month had lower rates of diabetes, high blood pressure and high cholesterol, although the risk of heart disease after such limited breast-feeding was comparable to that among mothers who had never breast-fed.

The research, which is to be published in the May issue of the journal Obstetrics & Gynecology, analyzed data on some 139,681 women who had enrolled in the Women’s Health Initiative, a long-term national study of postmenopausal women.

Women who reported a lifetime history of more than a year of breast-feeding were 20 percent less likely to have diabetes, 12 percent less likely to have hypertension, 19 percent less likely to have high cholesterol and 9 percent less likely to have had a heart attack or a stroke by the time they enrolled in the Women’s Health Initiative.

The new study’s chief author, Dr. Eleanor Bimla Schwarz, assistant professor of medicine at the University of Pittsburgh, said of breast-feeding, “We’ve known for a long time that it’s important for the baby’s health, but we now know it’s important for mothers’ health as well.”

Other experts cautioned, however, that while the study demonstrated an association between breast-feeding and health benefits, there was not necessarily a causal relationship. Women who breast-feed may simply lead more healthful lives than those who do not, these experts said, noting that the new analysis might not have been able to account for all the differences between the two groups.

Breast-feeders “may be healthier women who take better care of themselves,” said Dr. Nieca Goldberg, medical director of the N.Y.U. Women’s Heart Center.

“This is a nice association,” Dr. Goldberg said of the findings, “but we don’t know from the study what the physiological mechanism is.”

If there is such a mechanism, Dr. Goldberg suggested, it could lie in oxytocin, a hormone crucial to milk production. Oxytocin is known to relax blood vessels, she said, and may make them more flexible and more resistant to the buildup of plaque.

Breast-feeding is also known to play a role in healing after pregnancy, by causing uterine contractions that help restore the uterus to its original size more quickly. Further, women burn extra calories when making milk, helping them eliminate fat stores accumulated during pregnancy.

Other recent studies have suggested breast-feeding may also reduce the risk of osteoporosis and both breast and ovarian cancer, as well as Type 2 diabetes.