U.S. stocks opened mixed on Monday, as investors weighed disappointing service sector data that expanded less than forecast.

Economic data showed the Institute for Supply Management's non-manufacturing index came in at 53.0 in December, down from 53.9 the month before. Analysts had estimated the index would rise to 54.6. Separate data revealed new orders for U.S. factory goods rebounded in November, increasing 1.8 percent, the Commerce Department reported.

The Dow Jones industrial average rose 30.32 points, or 0.18 percent, to 16,500.31. The S&P 500 gained 1.73 points, or 0.09 percent, to 1,833.10. The Nasdaq Composite fell 13.13 points, or 0.32 percent, to 4,118.52.