The most convincing study I have come across was by Dr. Herbert Benson, a professor of medicine at Harvard Medical school, he tested 36 trancendental meditators and measured their heart rate, blood pressure and skin temperature. He found that when they meditated, they used 17% less oxygen, lowered their heart rates by three beats a minute and increased their theta brain waves - which exist prior to sleep - without slipping into the brain wave patterns of actual sleep.
Dr Benson stated that his research gives a viable biological explanation on techniques that people have been utilizing for millennia.

Scientific research with the use of brain-scans shows that meditation boosts the immune system,
and also suggests reduced depression, anxiety and stress due to re-wiring of the brain.

Dr Dean Ornish, found that meditation, along with yoga and proper nourishment, reversed the build up of plaque in coronary arteries. During a meeting of the American Urological Association, Dr Ornish announced his most recent findings that meditation may even slow prostate cancer.

Evidence from meditation researchers continues to mount. One study, for example, shows that females who meditate regularly and use guided imagery (visualization) have higher levels of the immune cells known to reduce tumors in the breast.

Meditation is being recommended by more and more physicians as a way to prevent, slow or at least control the pain of chronic diseases such as, heart conditions, AIDS, cancer, and infertility. Meditation is also being used to restore balance to psychiatric disturbances as depression, hyperactivity and the symptoms of attention deficit disorder (ADD & ADHD).