American researchers say they're shocked by how much information they unearthed about people simply by looking at the phone numbers they called and analysing the metadata.
The collection of metadata by America's National Security Agency (NSA) has been the subject of controversy since disclosures about it last year by former NSA contractor Edward Snowden.
The Stanford University study encouraged volunteers to install a tracking application called MetaPhone onto their phones, the ABC reports. Researchers collected information for several months, and say they were able to predict people's medical conditions, hobbies and relationships by only looking at the metadata.
Graduate student Jonathan Mayer, who led the study, says the results show a significant amount of personal information can be discovered through metadata.
"One of the things which is most concerning about the privacy properties we've uncovered is how easy it is to make inferences about the metadata on a large scale," he says. "We had a participant who...had calls with a lumber yard and a locksmith and a hydroponics dealer and a bong shop. (You) don't need a PHD in computer science to have some sense of what could be going on there."
Mr Mayer says his team's original hypothesis was that with only a few months of collection data and only a handful of users, the metadata would not be very revealing - but he was surprised.
"For some individuals, the fact that they have a particular medical condition is quite private, for others are the fact that they own a gun is quite private," he says.
"For others, their religious beliefs are quite private. And yet we were able to find many of these sorts of traits using just telephone metadata."
American president Barack Obama has reportedly indicated that he plans to seek little change to the NSA program that collects bulk metadata on domestic telephone calls.