According to a Goldman Sachs/ICSC tally, overall same-store sales fell 4.6 percent, worse than the 3 percent drop predicted.
The lower-than-expected results did not include Wal-Mart, which in recent months has boosted total results but has stopped reporting monthly figures.
Last month's same-store sales had edged up with Wal-Mart, but excluding the world's largest retailer, May was the 10th month of same-store sales declines, according to a tally by Goldman Sachs and the International Council of Shopping Centers.
In May, the Conference Board's Consumer Confidence Index rose to its highest level since September. And several retailers including BJ's and TJX Cos. indicated traffic improved in May.
But that generally did not translate to sales.
Results are a "clear indication that the consumer is not stampeding back to the stores, they're still being very careful," said BMO Capital Markets analyst John Morris. "I think the initial panic is over, but now the tough work begins. We're entering a slow summer period when there's not a lot to attract consumers into the stores."
Luxury chains and department-store operators continued to be the weakest sectors, with Saks Inc. and Neiman Marcus reporting double-digit declines. Discounters such as Ross Stores Inc. and teen apparel retailers such as The Buckle Inc. were stronger. Target reported a bigger drop than analysts expected, as apparel and home products continued to be weak sellers.
Overall, necessities such as food and health care products continue to be the strongest sellers.