An electronic medical records company called Practice Fusion has an offer that sounds too good to be true. Step 1: Download free software. Step 2: Collect $44,000.

While not quite so simple in practice, the offer is real, and it helps explain why Practice Fusion is shaking up the multibillion-dollar market for doctors’ record systems. In four years, the company has grown to serve 120,000 medical professionals who now are keeping records on 19 million patients. That represents about 8 percent of the U.S. population.

Until recently, doctors who wanted to install an electronic medical record (EMR) system could expect to pay tens of thousands of dollars. But now companies including Practice Fusion, Medgen, and AthenaHealth are pushing sharply discounted Web-based programs, akin to free e-mail services, in the hopes of building up valuable user bases.

Propelling the spread of low-cost EMR software are federal incentives, passed as part of the Obama administration’s economic stimulus package, that offer bonuses of between $44,000 and $64,000 to doctors, dentists, and even podiatrists who begin using electronic medical records for patients covered by Medicare and Medicaid.

Among EMR vendors, Practice Fusion has gone the furthest by cutting its price tag to zero. The company makes money by showing users banner ads from drug companies and other businesses. (The company will turn off the ads for a fee of $100 a month.) “It’s free in the same way that Google and Facebook are free,” says CEO and cofounder Ryan Howard.