It's his third day there, where's he receiving treatment for a recurring lung infection. At last report, Mandela was in serious but stable condition.
Mandela "once again endures the ravages of time in hospital," said the Cape Town based Desmond and Leah Tutu Legacy Foundation. "We offer our thanks to God for the extraordinary gift of Mr. Mandela, and wish his family strength."
The African National Congress, South Africa's ruling party, has dismissed a report in Monday's edition of The Star newspaper that Mandela's family had barred senior party leaders and government officials from visiting the hospital.
The 94-year-old last spent time in the hospital in April for pneumonia.
Mandela has been vulnerable to respiratory problems since contracting tuberculosis during 27 years as the prisoner of the white racist government. Most of that time was spent on Robben Island, off the coast of Cape Town, where Mandela and other prisoners spent part of the time toiling in a stone quarry.
He was freed in 1990, and then embarked on peacemaking efforts during the tense transition that saw the demise of the apartheid system and his own election as president in 1994.
The Associated Press contributed to this report.