The tomato pill had no effect on healthy volunteers whose blood vessels were already "normal".
Constriction of blood vessels reduces blood flow and is one of the main factors that can lead to heart attacks and strokes.
The study involved randomised trials to measure the function of blood vessels, called forearm blood flow, which is predictive of future cardiovascular risk. Thirty-six cardiovascular disease patients and thirty-six healthy volunteers were given either an off-the-shelf supplement containing 7mg of lycopene called Ateronon or a placebo.
The patients with cardiovascular disease were all on cholesterol-lowering statins. However, despite this, they still had a relatively impaired function of the endothelium - the inner lining of blood vessels - compared with healthy volunteers.
This function is determined by the response of blood vessels in the forearm to a naturally occurring molecule called acetylcholine.
Endothelial function predicts future events, so having a healthy endothelium is an important factor in preventing the evolution of heart disease.
Lead scientist Dr Joseph Cheriyan, from Addenbrooke's Hospital and Cambridge University, said: "We've shown quite clearly that lycopene improves the function of blood vessels in cardiovascular disease patients.
"A daily 'tomato pill' is not a substitute for other treatments, but may provide added benefits when taken alongside other medication. However, we cannot answer if this may reduce heart disease - this would need much larger trials to investigate outcomes more carefully."
Professor Jeremy Pearson, associate medical director at the British Heart Foundation, said: "Further work is needed to understand whether the beneficial effects seen in this small study translate into clinical benefit for at-risk patients."