The customer went into a store on the city's far west side Tuesday morning, police said, and asked if it stocked the sleeveless shirts in size 3XL, WLS-TV, Chicago reported.
The clerk, 51, said the story didn't carry that size. Police say that's when the customer pulled out a baseball bat and attacked him.
The report did not give the name of the clerk or the store, and police said no arrests have been made as yet.
The clerk was was released after being treated at Our Lady of the Resurrection Medical Center for head wounds.