The Pending Home Sales Index fell further than expected. Economists had predicted a drop of 0.1 percent.
December's drop put the index at 96.6, down from November's 100.1.
From December 2010, the index is up 5.6 percent, the NAR said.
"Housing affordability conditions are too good to pass up. Our hope is lending conditions will gradually improve with sustained increases in closed existing-home sales," said NAR Chief Economist Lawrence Yun.
Region by region, the index fell 3.1 percent in the Northeast and rose 4 percent in the Midwest. In the South, the index dropped 2.6 percent. In the West, the index fell 11 percent, the NAR said.