More than half all deaths have no recorded cause, making effective health monitoring and policymaking far more difficult, the World Health Organization says.
However improved collection of statistics meant 27 million of the world's 56 million estimated deaths in 2015 were registered with a cause compared with only about a third in 2005, according to the UN health agency's latest global health report.
Several countries including China and Turkey had made "significant strides" in data collection, the WHO said on Wednesday.
In Iran, it said, 90 per cent of deaths are now recorded with details of causes, compared with 5 per cent in 1999.
While things have improved significantly in recent years, many countries still do not routinely collect high-quality health data, the WHO's assistant director-general for health systems and innovation, Marie-Paule Kieny, said.
"If countries don't know what makes people get sick and die, it's a lot harder to know what to do about it."
The WHO is working with countries to boost health information systems and improve data quality, she said.
This year's WHO report focused on the UN Sustainable Development Goals, a set of internationally agreed targets which track issues such as health, climate, sanitation and economic inequality.
It found that while maternal and newborn death rates are declining, the 2015 global neonatal mortality rate was 19 per 1000 live births and the under-five death rate was 43 per 1000 live births.
About 830 women died every day due to complications of pregnancy or childbirth in 2015, it said.
Looking at infectious diseases, it found an estimated 2.1 million people were newly infected with HIV in 2015, 35 per cent fewer than in 2000.
There were an estimated 212 million malaria cases globally in 2015.
