The fabulous routine was caught on camera and posted to Vimeo where it has been viewed more than three million times.
The spectacular event was organised by Lauren Keppel as a 56th birthday surprise for her mum Amy, whose ovarian cancer was diagnosed as terminal last year.
The flashmob begins with just three members of Amy's immediate family dancing to The Emotions' 1977 hit Best of My Love at the family's home in Wheaton, near Chicago.
It quickly escalates as more and more family and friends join the performance.
Lauren said: "The original intent of this flashmob was simply to remind my mom of how surrounded by love she is.
"We recorded it so that she could re-watch it in times of struggle and so that the 50 of us who participated could watch her beautiful reactions once she's no longer with us.
"The fact that the video has travelled the globe has been incredible and overwhelming, we are so grateful to be able to share the essence of my mom's love with the world."