Unification Church founder Sun Myung Moon is being treated for pneumonia in an intensive care unit at a hospital in Seoul, South Korea.
Douglas Burton, a US-based spokesman for the church, said he received a memo about the Rev Moon's condition.
The memo said the 93-year-old "was pushing his limits in carrying out his schedule" when he fell ill.
It saids his wife and children were by his bedside.
Mr Moon is a self-proclaimed messiah who founded the Unification Church in Seoul in 1954.
The church, whose followers have been dubbed Moonies, gained fame in the 1970s for holding mass weddings of thousands of followers.
Its business empire includes The Washington Times newspaper and the New Yorker Hotel in Manhattan.