Archbishop Christodoulos, 68, leader of the Orthodox Church in Greece was admitted to hospital for tests on Monday for re-evaluation of his cancer treatment. His office said it was unsure how long he would be hospitalised.
The Archbishop was diagnosed with cancer in June and for ten weeks he was hospitalised in Miami until a planned liver transplant was cancelled when it was discovered that the cancer had spread.
Archbishop Christodoulos was elected as leader of the Church in 1998 and was instrumental in moves to improve relations with the Roman Catholic Church.
In 2001, despite the protests of Orthodox hard liners, Archbishop Christodolous welcomed Pope John Paul II to Greece. It was the first time a pope had visited Greece in almost 1,300 years.
In another gesture the Archbishop visited the Vatican last year where he signed a joint declaration with Pope Benedict XVI calling for inter-religious dialogue and stating again both Churches' opposition to abortion and euthanasia.
Almost all of Greece's nearly 11 million people are baptised as Orthodox. Overall there are 200 million Orthodox Christians across the globe, making it the second largest denomination after the Roman Catholic Church.
