The 86-year-old is one of only three surviving leaders of the radical movement who are facing war crimes charges in a trial by a U.N.-backed court.
Tribunal spokesman Neth Pheaktra says Nuon Chea was hospitalized Sunday.
Pheaktra said Monday that hearings in Nuon Chea's case had been postponed.
Nuon Chea was known as Brother No. 2 after Pol Pot, the deceased leader of the extremist group whose fanatical efforts to realize a utopian society during the Khmer Rouge's 1975-79 reign led to the death of some 1.7 people from starvation, diseases, overwork and executions.
Nuon Chea is charged with crimes against humanity, genocide, religious persecution, homicide and torture.