Women over 60 and men over 50 living alone had an increased risk of conditions including severe angina and heart attacks.
The research appears in the Journal of Epidemiology and Community Health, according to BBC News.
However, heart experts said habits such as smoking and a poor diet, more common among those who live alone, were the reasons for the increased risk.
Factors associated with the lowest risks included living with a partner, a high level of education, and being in work.
Women who were divorced also had a lower heart disease risk.
The research is based on a three-year study of more than 138,000 adults aged between 30 and 69 in one area of Denmark. Heart disease was diagnosed in 646 people during that period. Researchers established that a poor education and living on a pension were associated with an increased risk of the syndrome, but age and living alone were the main predictive factors, Times Online reports.
Women older than 60 and men older than 50 who lived by themselves were twice as likely to develop heart disease. Researchers said, however, that smoking, obesity, high cholesterol and fewer visits to the doctor tended to be more common in those who lived alone.