The study, which followed almost 2,600 older adults for a decade, found that those who drank diet soda every day were 44 percent more likely than non-drinkers to suffer a heart attack or stroke.
The findings, reported in the Journal of General Internal Medicine, don't prove that the sugar-free drinks are actually to blame.
There may be other things about diet-soda lovers that explain the connection, researchers say.
"What we saw was an association," said lead researcher Hannah Gardener, of the University of Miami Miller School of Medicine. "These people may tend to have more unhealthy habits."
Daily diet-soda drinkers did tend to be heavier and more often have heart risk factors like high blood pressure, diabetes and unhealthy cholesterol levels.
That all suggests that people who were trying to shed pounds or manage existing health problems often opted for a diet soda over the sugar-laden variety.
But even after the researchers factored in those differences -- along with people's reported diet and exercise habits -- they found that daily diet soda was linked to a 44-percent higher chance of heart attack or stroke.
The findings are based on 2,564 New York City adults who were 69 years old, on average, at the outset. Over the next decade, 591 men and women had a heart attack, stroke or died of cardiovascular causes.
That included 31 percent of the 163 people who were daily diet-soda drinkers at the study's start. In contrast, 22 percent of people who rarely or never drank diet soda went on to have a heart attack or stroke.
There was no increased risk linked to less-than-daily consumption. Nor was regular soda tied to heart attacks and strokes.