Stanford University said that 11,000 iPhone owners signed up for a heart health study using Apple’s newly-announced ResearchKit in the first 24 hours–completely unprecedented numbers.

“To get 10,000 people enrolled in a medical study normally, it would take a year and 50 medical centers around the country,” said Alan Yeung, medical director of Stanford Cardiovascular Health, speaking to Bloomberg.

Stanford is one of five academic centers that have developed apps that use the iPhone’s built-in accelerometers, gyroscopes and GPS to provide data which assists in medical research. There are, say researchers, both pros & cons to recruiting study participants through ResearchKit … 

The big advantage is that researchers no longer have to rely on what study participants claim about things like the amount of exercise they take: the iPhone reports the actual data.

The iPhone helps address a problem that standard trials often encounter: people enrolled in studies often falsely report their activity to researchers. By using its internal components or secondary devices connected wirelessly via Bluetooth, the iPhone can silently measure users’ behavior, without relying on them to keep track or be honest about what they’re doing.

But there are also downsides. For example, iPhone users may not be representative of the U.S. population as a whole.

The average iPhone user is more likely to have graduate and doctoral degrees than the average Android user, and has a higher income as well, according to polling company CivicScience Inc. Those sort of demographic differences could skew the findings from a study.

Participants may also accidentally select options when answering questions through the app, or have their responses biased by being presented with multiple-choice responses rather than a human being asking open-ended questions.

These difficulties aside, the potential seems huge. Todd Sherer, CEO of the Michael J. Fox Foundation for Parkinson’s Research, says that a traditional study with 800 participants over five years cost around $60M. The Parkinson’s app had 5,589 participants by Tuesday morning.

If you want to know more about how ResearchKit works, and how you can help future medical studies, check out our hands-on guide.