Plumbers Union Local 130 carried out the tradition, which has been in place for 50 years, CBS Chicago reported.
Organizers said the dye formula is a closely held secret.
Hillsborough River was also dyed green Saturday during the Mayor's River O'Green Fest in Tampa, Fla., the Tampa Bay Times reported.
And in Boston, hundreds of people packed into the Boston Convention and Exhibition Center for the annual South Boston St. Patrick's Day Breakfast.
St. Patrick's Day in Philadelphia was less celebratory when a New Jersey-bound PATCO train, filled with drunken party-goers, was disabled on the Benjamin Franklin Bridge Saturday at 6 p.m., the Philadelphia Inquirer said.
A second train picked the passengers at 7:30 p.m., PATCO spokesman Tim Ireland said.