Judges at Cambodia's Khmer Rouge war crimes court suspended proceedings on Monday as one of the three former leaders on trial for alleged atrocities was hospitalised.
"Brother Number Two" Nuon Chea, 86, was taken to hospital on Sunday afternoon.
According to a medical report read to the court, Nuon Chea was diagnosed with "acute bronchitis" and doctors needed between four and seven days to monitor his condition.
"I cannot say how serious he is, but he has been placed in the intensive care unit," tribunal spokesman Neth Pheaktra told AFP.
Nuon Chea's lawyer Son Arun said his health was "deteriorating because he has many illnesses".
"He will not waive his rights to attend the hearing because this case is very important. All witnesses who are testifying are important for him," he said.
Nuon Chea and two other top former Khmer Rouge leaders deny charges including war crimes and genocide from their roles in the deaths of up to two million people.
Health fears have long hung over the UN-backed tribunal with the octogenarian defendants all suffering from varying ailments.
The 1975-1979 government oversaw one of the worst horrors of the 20th century, wiping out nearly a quarter of the population through starvation, overwork and execution.