The Chicago Purchasing Managers' Business Barometer fell to 58.3 in December 60.8 in November, the National Association of Purchasing Management-Chicago said Wednesday.
The data is compiled on a seasonally adjusted basis. An index reading below 50 signals a slowing economy, while a level above 50 suggests expansion.
Economists polled by Thomson Reuters predicted a 60.4 reading for the index.
The decline took the index to a five-month low.
"The slowdown in the pace of activity exhibited since October's one year high of 66.2 has been marked, although the Barometer averaged 61.8 in Q4, a little higher than the 59.1 seen in Q3," the group said in a release. "For 2014 as a whole the Barometer put in the best performance in three years, averaging 60.8 compared with 56.1 in 2013 and 54.6 in 2012."
