Walmart shook up the health care world earlier this month when it began flirting with making a serious move into retail clinics, store-based sites where the retail giant could treat patients with common and chronic conditions.

Walmart’s announcement surprised many, but it was also part of a larger trend: The number of retail clinics has risen steadily for the past five years. And new data show that patients are starting to show up in droves: The number of patient visits to retail health clinics grew 10-fold in just the past two years, according to research this month from the RAND corporation.

The RAND study breaks down who actually comes to these clinics. The biggest determinant is distance: People who live within one mile of a retail clinic were 7.5 times more likely to visit one than people who lived 20 miles away. Retail clinic patients also tend to have slightly higher incomes and to be female. Here’s how all that looks in graph form:

Health policy experts I’ve spoken to about retail clinics see their rise as a disruptive innovation, one that stands to change the way health care is delivered in the United States. The RAND study shows that for some Americans, that’s already happening. “As a less costly alternative to primary care physician or emergency room visits, health plans increasingly are covering care at a retail clinic,” Deloitte’s Paul Keckley noted in a 2009 report on the issue.

One disappointing finding in the RAND study had to do with patients of retail clinics in federally designated Health Professional Shortage Areas, places the government deems to have too few medical providers. There has been some hope that, with retail giants like CVS and Walmart being more ubiquitous than doctor’s offices, they could help bridge that gap in medical care. The study found no association, however, between primary care physician availability and retail clinic use.