FORTUNE — This morning we put Jazz Pharmaceuticals JAZZ atop our annual list of the world’s fastest-growing public companies, with three-year average revenue and profit growth of 68% and 279%, respectively. So I was curious to see how Jazz had worked out for Kohlberg Kravis Roberts & Co. KKR , which led a $250 million investment into the company nine years ago.

It was the first time that KKR ever played venture capitalist, and arguably the last (depending on how you view its 2012 investment in wireless stereo company Sonos). It also was the private equity giant’s initial foray into biotech.

KKR led the Series B round with a $130 million investment, alongside smaller investments from four other private equity firms and Jazz’s existing VC backers. At the time, Jazz was really just a well-capitalized platform in search of product. The Palo Alto, Calif.-based company had been founded one year earlier by a group of executives at Alza Corp. (acquired by Johnson & Johnson in 2001), with plans to acquire and commercialize drugs for treating neurological and psychiatric disorders. It never raised more private funding, instead going public in 2007 with two FDA-approved products and another six in varying degrees of development.

The IPO price was $18 per share, with KKR holding onto all its stock. For a while, it looked like a lousy decision. with Jazz trading at a low point of just $0.60 per share by April 2009. But KKR kept the faith, even buying more shares that September via a direct placement.

Its first sale was a small chunck in December 2010 that netted less than $1 million. But then it sold nearly one-third of its stake in March 2012 and an even larger block in March 2013. All together, KKR has generated $366 million for its Jazz Pharma sales. More importantly, it still holds a 5.5% ownership stake that is currently valued at around $278 million ($271m when market opened, but the stock is up more than 2% so far).

That means that KKR has made over half a billion dollars off of Jazz Pharmaceuticals. The 4.7x cash-on-cash multiple isn’t exactly what most traditional VCs would consider a grand slam, but most traditional VCs also wouldn’t invest $130 million into a pre-revenue biotech play…

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