Mandela, the 94-year-old former president and anti-apartheid leader, had a recurrence of pneumonia, said presidential spokesman Mac Maharaj. South African officials had previously not specified that Mandela had pneumonia, saying instead that he had a lung infection.
Mandela's medical team reported that the increasingly frail ex-leader "had developed a pleural effusion which was tapped," Zuma's office said in a statement. "This has resulted in him now being able to breathe without difficulty. He continues to respond to treatment and is comfortable."
The president's office thanked all who have prayed for Mandela and his family and have sent messages of support.
Mandela was admitted to a hospital near midnight Wednesday night in the capital, Pretoria. It was his third trip to a hospital since December, when he was treated for a lung infection and also had a procedure to remove gallstones. Earlier this month, he spent a night in a hospital for what officials said was a scheduled medical test.
Mandela became South Africa's first black president in 1994 after elections were held, bringing an end to the system of white racist rule known as apartheid. He had spent 27 years in prison under the apartheid regime and after his release in 1990 was widely credited with averting even greater bloodshed by helping the country in the transition to democratic rule.