From Cancer.org:

Studies that look at people and their habits have linked vegetarian diets with a decreased risk of heart disease, diabetes, high blood pressure, obesity, and colon cancer. A review of studies looked at the effects of vegetarian diets among Seventh-Day Adventists, whose religious doctrine advises against eating animal flesh. The review found that Seventh-Day Adventists had less heart disease and fewer cases of some types of cancer than most people. For instance, vegetarians tended to have lower rates of prostate and colon cancer. On average, Seventh-Day Adventist males had serum cholesterol levels and blood pressures that were lower than average. And, their overall cancer death rate was about half that of the general population. The overall cancer death rate of females was also lower. The report cautioned that abstinence from tobacco and alcohol was very likely responsible for some of the health effects that are often credited to vegetarian diets in Seventh-Day Adventists.

A study that looked at groups of people in Germany found the death rate for colon cancer was lower among moderate and strict vegetarians compared with that of the general population. The authors of this study also noted vegetarians tend to be more health conscious than average. In Great Britain, a 17-year study followed 11,000 vegetarians and health-conscious people. They concluded that eating fresh fruit every day was linked to a significant reduction in deaths from ischemic heart disease, stroke, and all causes of death combined. Another population study found men who ate a diet rich in grains, cereals, and nuts had a lower risk of prostate cancer.

In 1991, two nutritionists studying the benefits and risks of vegetarian diets reported that vegetarians are not necessarily healthier than non-vegetarians. They found that well-planned omnivorous diets (which include meats) can provide health benefits as well. The nutritionists also pointed out that many vegetarians adopt a healthier lifestyle, including more physical exercise and not smoking. These factors would likely improve the overall health of vegetarians and account for part of the health benefit that was first thought to be due to their diet.

To look at these other health factors, a study published in 2005 compared more than 1,000 German vegetarians with nearly 700 health-conscious non-vegetarians over a 21-year period. This study found that there were no major differences between the groups in terms of death and disease, although the vegetarians had slightly less heart disease. Both groups were healthier than the general population, in part due to less smoking and more physical activity.

Most human evidence about vegetarianism consists only of studies that observe people (observational studies) and their risk for various diseases such as cancer. These studies don't test different diets; they only look at what people are already doing. Because of this, the studies often can't control for non-food differences (like exercise and other healthy habits) between vegetarians and other people.

Very few clinical studies have been reported in which people are put on different diets and studied over time. A few studies of men with prostate cancer have reported that major life changes including vegetarianism, exercise, and stress reduction can slow the rise in blood PSA levels. How much the vegetarian diet contributed to these benefits remains unproven.