Silicon Valley health IT startup Castlight Health (@castlighthealth) announced it has closed a $100 million Series D round, bringing their total capital raised to $181 million, leaving it poised for acquisitions or an IPO.

Castlight has been pioneering the use of price transparency and comparison tools in health care, offering a B2B service that enables self-insured businesses to provide their employees with tools to compare costs and quality of a wide range of tests and procedures across providers.

Castlight even counts provider Cleveland Clinic among its financial backers, and the Wall Street Journal has called Castlight the #1 venture backed business of 2011.

The latest round was led by two “major” mutual funds, with contributions from T. Rowe Price, and Redmile Group. Castlight’s previous investors include Morgan Stanley, Wellcom Trust, US Venture Partners, Maverick Capital, Oak Investment Partners, Venrock, among others.

The company has remained largely under the radar, focusing on its ten or so core clients, which include Honeywell, Life Technologies, insurance broker Willis North America, and grocery store chain Safeway. The company charges its employer clients by employee per month.

“As the recognized market leader, we know the vast potential for our health care transparency solutions,” said Giovanni Colella, M.D., co-founder and CEO of Castlight Health. “Castlight Health has achieved tremendous growth in a short period of time and currently maintains a strong cash position. By expanding our funding, we can respond to the growing demand for our solutions, seize major market opportunities, increase the breadth of our offerings and extend our reach. We believe there is a huge opportunity to better engage companies and their employees as they make health care decisions and the latest round of funding will allow us to further innovate the way health care is consumed.”

Speculation is that the next step for the company will be an IPO, though perhaps not in the immediate future. According to Forbes, the company still has most of the $81 million raised in the company’s first three fundraising rounds in the bank, which leads me to believe this huge raise will be used to execute on strategic acquisition opportunities. The company will also probably add to its staff of approximately 100 and expand its footprint well beyond Silicon Valley.

“The health care industry is in dire need of innovation,” said Bryan Roberts, partner at Venrock and co-founder and chairman of Castlight Health. “Castlight has the opportunity to dramatically improve the efficiency of the U.S. health care system, providing employers and consumers with the information required to make good cost and quality health care decisions. Over the last two years, Castlight Health has created the unique products to drive customer traction and ROI, which sets them on a terrific growth trajectory over the next several years.”

TechCrunch puts this down as one of the largest venture investments in a health IT company to date, which is probably correct. Another interesting way to look at the staggering fundraising round is to divide it by their total number of followers on Twitter, arguably a decent way to gauge the company’s digital influence, which comes out to $148,809/follower.

To put this in context, even if Facebook is able to raise $10 billion dollars in its IPO later this month, that would only break out to be approximately $12.50 for each registered user.

“Traditionally, individuals have been very passive participants in managing their health care, as they have no insight into the cost and quality of the services,” said Dr. Toby Cosgrove, CEO of the Cleveland Clinic. “Transparency in health care can lead to higher-quality, lower-cost health care, as well as more consumer engagement. With its innovative technology and strong understanding of the market, Castlight is well positioned to make health care transparency a reality and transform how users make decisions.”