The Chicago Purchasing Managers' Business Barometer dropped to 45.8 in February from 59.4 in January, the National Association of Purchasing Management-Chicago said Friday.
The 45.8 is the lowest level for the index since July 2009 and the first contraction since April 2013.
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 58.0 reading for the index.
"The West Coast port strike and the harsh winter probably had a negative impact in February, although it is difficult to gauge the magnitude," the group said in a release. "It's too early to conclude that February represents a change in the relatively strong trend seen recently. Nonetheless, the weakness in the Barometer points to softer GDP growth over the first quarter than previously expected."