From 8:30 p.m. to 9:30 p.m. local time around the globe lights were turned off in landmarks, businesses and homes, saving energy as a symbol to support efficiency and ecology.
Hundreds of other buildings, institutions and landmarks throughout the Chicago area -- including Chase Tower, the James R. Thompson Center and Willis Tower -- planned to take part.
Earth Hour is observed on the last Saturday of March when everyone is asked to turn off all non-essential lights for an hour.
The annual event has grown from a one-city initiative to a global movement with more than 5,200 cities becoming involved since 2007, officials said in a news release.
Earth Hour, organized by the World Wildlife Fund, has reached about 1.8 billion people in 135 countries globally, officials said.