What: Shares of Priceline Group (BKNG 2.44%) 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.