They were among 10 people who were part of a cold weather swim group doing a polar-plunge-type activity. The group went into the water at about 9 a.m. near Park Avenue Beach, Highland Park officials said.
Upon entry to the lake, authorities said several people were struggling to get out of the water, prompting an employee of the North Shore Yacht Club to call 911, according to the U.S. Coast Guard, which was assisting in the search.
A 46-year old Itasca man -- identified as Stanislaw Wlosek, Jr. -- was pulled out of the water unresponsive and taken to Highland Park Hospital, where he died, officials said.
Anyone who has spotted the missing swimmer is asked to call the Highland Park Police Department at 847-432-7730.
Lake Michigan had temperatures in the upper 40s to low 50s on Sunday morning, according to the National Weather Service.