A study, presented to the European Society of Cardiology (ESC), suggests that polyphenols - which occur naturally in cocoa - may help to maintain cardiovascular health.
The findings follow suggestions that a compound in cocoa beans can protect against tooth decay.
Research in the laboratory has shown that cocoa polyphenols appear to relax the blood vessels, thus making blood flow more efficient and reducing the strain on the heart.
There is also evidence to suggest that cocoa polyphenols act like antioxidants.
Antioxidants, also found in fruit and vegetables, have been shown to prevent damage to body tissues.
They help to reduce the effect of LDL, or "bad", cholesterol which has been implicated in coronary heart disease.
It is also thought that cocoa polyphenols may help reduce the risk of formation of blood clots.
Regular intake
In addition, the main fat in chocolate, stearic acid, which has been shown to exert a neutral effect on blood cholesterol.
Professor Carl Keen, of the University of California, said: "We have already conducted preliminary research and are finding more evidence to suggest that regular intake of active cocoa components may contribute to a lower risk of blood clots within our blood vessels.
"These results lead us to believe that chocolate may contribute to a healthy, well-balanced diet. "
Although cocoa polyphenols are naturally occurring components in cocoa, manufacturing processes, such as fermentation and roasting, can affect the levels of cocoa polyphenols that are in the final chocolate product.
The researchers said positive effects were noticed after eating a chocolate bar weighing 35g.
However, a spokesman for the British Heart Foundation (BHF) said more studies were needed before chocolate could be promoted as a healthy food.
Cathy Marshall, a BHF cardiac nurse adviser, said: "Small amounts of dark chocolate may offer some protection against coronary heart disease, but large amounts are not a good idea - they can lead to increases in blood tri-glycerides and glucose.
"All the heart benefits of cocoa can be gained by eating a diet rich in fruit and vegetables.
"The message is: enjoy chocolate sparingly but those who are diabetic or overweight should avoid it all together and eat five portions of fruit and vegetables daily instead."
Some of the research was funded by the Mars company.