Paramedics arrived and did CPR, then everyone was told to leave the building. Fans described a chaotic scene, and say many were angry.
"A lot of people were upset...not being disrespectful, but I paid and I only got half a show," Simon Massey said.
The Medical Examiner told TODAY'S TMJ4 the victim died at the hospital of apparent natural causes.
Pabst Theater Group, which operates the Riverside Theater, released the following statement about the incident:
"We are saddened by the tragedy that happened at the Riverside Theater last night. Our deepest sympathy and condolences go out to the family and we ask the public to respect their privacy. We appreciate the efforts of the Milwaukee Police Department, Milwaukee Fire Department and Per Mar Security in quickly responding and assisting during this difficult time."
Check back often for updates on this developing story.