The Chicago Purchasing Managers' Business Barometer gained to 65.5 in May from 63.0 in April, the National Association of Purchasing Management-Chicago said Friday.
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 61.0 reading for the index.
"The Barometer's faster expansion was helped by a return to normality following the adverse weather conditions of Q1, with companies playing catch-up to make up for lost output," according to a press release. "This was clear from the large spike in Order Backlogs to a three year high, as companies faced a double whammy of strong new demand as well as having to shift existing orders."