The head of the Greek Orthodox Church, Archbishop Christodoulos, is undergoing liver transplant surgery in the US, church officials say.
Archbishop Christodoulos was diagnosed with cancer in June
The operation, at a hospital in Miami, Florida, is expected to last for up to 10 hours.
Archbishop Christodoulos, 68, was diagnosed with cancer in June after intestinal surgery.
He travelled to Miami in August, but had to wait until Sunday for a compatible liver donor.
He was elected Archbishop of Athens and all Greece in 1998.
Last year, he met the Pope at the Vatican as part of efforts to bring the two churches together - the first such talks between Greece's most senior cleric and the leader of the world's Roman Catholics.
Their meeting focused on attempts to end the Great Schism that dates from 1054.
The Greek Orthodox Church is an autocephalous, or ecclesiastically independent, branch of the Eastern Orthodox Communion, covering the territory of Greece.