The chief executive officers of large U.S. companies are much more optimistic about the economy than a few months ago, although their expectations for growth remain modest, according to a quarterly survey by the Business Roundtable.

The Roundtable's economic outlook index jumped to 101, its highest level since the first quarter of 2006 and up from 86 three months ago. At its record low in early 2009, the index stood at negative-5.

More CEOs than last quarter -- 80 percent -- expect their sales to increase over the next six months, and about six out of 10 expect to boost capital spending. Fewer than half -- 45 percent -- will hire more workers, but that figure is up from 31 percent three months ago.

The Roundtable, an association of U.S. CEOs who run companies with about $6 trillion in annual sales and 12 million workers, polled its members in November and the first few days of December.

The executives' improved confidence comes despite modest expectations for U.S. economic growth. On average, business leaders expect the economy to expand by 2.5 percent in 2011, only slightly better than the 1.9 percent expansion anticipated for 2010.

The Roundtable also asked members about cost pressures. It found 29 percent were concerned about materials like copper and other metals, as well as healthcare and labor costs.

(Reporting by Nick Zieminski; Editing by Lisa Von Ahn)