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.
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.
Researchers from Aarhus Sygeus University studied population data on more than 138,000 adults living in the Aarhus area.
Between 2000 and 2002, 646 people were diagnosed with severe angina, or sustained a heart attack, or died because of a severe heart problem, a range of conditions known as acute coronary syndrome.
Age and living alone were the strongest factors for predicting someone would develop one of the conditions.
Women aged over 60 who lived alone made up just over 5% of the population, but accounted for a third of all deaths from the syndrome within 30 days of diagnosis.
Lone men over 50 comprised just under 8% of the population, but accounted for two thirds of deaths from the syndrome within 30 days of diagnosis.
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.
Smoking, obesity and high cholesterol are all more common among those who live alone.
This group is also less likely to have a social support network to draw on, and is likely to make fewer visits to the family doctor.
Dr Kirsten Nielsen, who led the research, said: "There is an accumulation of heart disease risk factors in people who live alone.
"But something else is also happening, which we haven't yet identified.
"Studies on monkeys have shown animals who are isolated have a higher risk of atherosclerosis [a build up of fatty deposits in the arteries].
"Living alone is a risk factor GPs should take into account - and it may be necessary to contact those living on their own and advise them on how to cut their risk of disease."
Ellen Mason, spokesperson for the British Heart Foundation said: "Lifestyle habits linked to living alone, such as smoking and eating unhealthy food, are more of a problem than living alone itself.
"The weakness of the researchers' interpretation is that many people who choose to live alone have strong social support and healthy lifestyles, all of which help keep their heart healthy."Last Mod: 20 Eylül 2018, 18:16