What: Shares of Priceline Group (NASDAQ:PCLN) were up 10.7% as of 11:00 a.m. EST Monday after the online travel specialist reported strong fourth-quarter 2015 results.

So what: Quarterly revenue rose 8.7% year over year, to $2.0 billion, driven by a 14.4% increase in agency revenue, to just more than $1.4 billion. The company reported a 6.8% decline in merchant revenue, to 445.8 million, and a 17% increase in revenue from advertising and other sources, to $153.5 million.

Gross bookings rose 12.7% year over year -- 24% on a constant-currency basis -- to $12 billion. Properties at booking.com also grew 34% year over year, to more than 850,000. That translated to 11% growth in adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA), to $790 million, an 11.1% increase in adjusted net income, to $641 million, and 16.4% growth in adjusted net income per share, to $12.63.

Analysts, on average, were anticipating lower revenue of $1.96 billion, and adjusted earnings of $11.80 per share.

Priceline Group CEO Darren Huston noted Priceline enjoyed accelerated 27% growth in hotel room nights booked during the quarter, to 99 million, then elaborated: "The Group's brands are starting 2016 with strong momentum. We will continue to invest in our brands to organically grow for the long term using our consistent ROI-driven investment philosophy of creating value."

Now what: For the current quarter, Priceline expects revenue will increase 9% to 16%, or to a range of roughly $2 billion to $2.13 billion, while adjusted EBITDA should be in the range of $580 million to $630 million. Adjusted net income per share should be between $9.00 and $9.60. By comparison, analysts' consensus estimates called for first-quarter revenue of $2.07 billion, and slightly higher adjusted earnings of $9.61 per share.

Similar to last quarter, however, Priceline has long demonstrated a propensity for under promising and over delivering on guidance. In the end, with shares still trading well off their 52-week-highs set in early November, Priceline investors have every right to celebrate today's solid report.

Steve Symington has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Priceline Group. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.