German researchers followed nearly 20,000 people over eight years, sending them several questionnaires about their diet and exercise habits.
They found people who had an average of six grams of chocolate per day - or about one square of a chocolate bar - had a 39 percent lower risk of either a heart attack or stroke. The study is scheduled to be published Wednesday in the European Heart Journal.
Previous studies have suggested dark chocolate in small amounts could be good for you, but this is the first study to track its effects over such a long period of time. Experts think the flavonols contained in chocolate are responsible. Flavonols help the muscles in blood vessels widen, which leads to a drop in blood pressure.
"It's a bit too early to come up with recommendations that people should eat more chocolate, but if people replace sugar or high-fat snacks with a little piece of dark chocolate, that might help," said Brian Buijsse, a nutritional epidemiologist at the German Institute of Human Nutrition in Nuthetal, Germany, the study's lead author.
The people tracked by Buijsse and colleagues had no history of heart problems, had similar habits for risk factors like smoking and exercise, and did not vary widely in their Body Mass Index.
Since the study only observed people and did not give them chocolate directly to test what its effects were, experts said more research was needed to determine the candy's exact impact on the body. The study was paid for by the German government and the European Union.
Doctors also warned that eating large amounts of chocolate could lead to weight gain, a major risk factor for heart problems and strokes.
"Basic science has demonstrated quite convincingly that dark chocolate ... improves vascular and platelet function," said Frank Ruschitzka in a statement, a cardiologist at the University Hospital Zurich and spokesman for the European Society of Cardiology. "However, before you rush to add dark chocolate to your diet, be aware that 100 grams of dark chocolate contains roughly 500 calories."
Buijsse said people hoping to benefit from chocolate's heart healthy effects should cut out other snacks and sweets if they wanted to eat chocolate. "The biggest problem with this is not to gain weight," said Buijsse. "Eating too much chocolate will have negative effects that far outweigh the positive effects of cocoa."
By AP Medical Writer Maria Cheng