Bill Haley had asked his neighbor in Van Dyke Village in New Castle for the box and received permission to take it, The (Wilmington) News Journal reported. A shell inside the box exploded when he opened it.
About 30 homes in the area were evacuated.
Michael G. Chionchio, an assistant state fire marshal, said ammunition experts from Dover Air Force Base found 11 more shells in the box.
The woman who put it out for a special trash pickup was recently widowed and had decided to clear out some of her late husband's belongings, authorities said. She was unaware of the contents of the box.
Karen Johnstone, whose parents live across the street from Haley, said her father, Michael McLain, rushed over when he heard the explosion and performed first aid.
"My dad said he had a wound to his leg and then he had blood coming out of his ears," she said. "He said he had cuts everywhere from the shrapnel."
Haley was reported to be in critical but stable condition at Christiana Hospital.