Cancer is now the leading cause of death in high-income countries, while cardiovascular disease is still the biggest killer in poorer nations.
Scientists say deaths from cancer in richer countries have become twice as frequent as those from cardiovascular disease.
When looking at all causes of death except cancer, the overall mortality per 1,000 person-years was lowest in high-income countries, at 3.4%.
It was highest in low-income countries, at 13.3%.
Person-years is a measurement that takes into account both the number of people in a study, and the amount of time they spend in the study.
Cardiovascular disease was the most common cause of death overall at 40%, but that ranged from just 23% in high-income countries to 43% in low-income countries.
The research was published in The Lancet, and involved 162,534 adults aged 35 to 70 from 21 countries.
They were followed for an average of nine-and-a-half years.
"The fact that cancer deaths are now twice as frequent as CVD deaths in high-income countries indicates a transition in the predominant causes of death in middle age," said Professor Salim Yusuf, the principal investigator of the study.
"As CVD declines in many countries because of prevention and treatment, cancer mortality will likely become the leading cause of death globally in the future."
Prof Yusuf added that the results are likely to be applicable to other countries with similar economic and social characteristics and health care.