Researchers in Italy gave a daily omega-3 supplement to nearly 3,500 patients and a placebo (dummy pill) to a similar number of volunteers.
They found that patients who increased their intake of omega-3 were slightly less likely to die than those who took placebo pills.
According to experts, a similar effect could be achieved simply by increasing consumption of oily fish.
Dr Richard Bonow, chief of cardiology at Northwestern University Hospital in Chicago and a former president of the American Heart Association, told the Associated Press: "It's a small benefit, but we should always be emphasising to patients what they can do in terms of diet that might help."
UK health experts recommend that we eat a portion of oily fish every week; as well as omega-3, this provides proteins, vitamins and minerals.