Lugo, who suffers from malignant lymphoma, has temporarily transferred power to Paraguayan Vice President Federico Franco.
He arrived late Saturday at the prestigious Hospital Sirio Libanes in Sao Paulo with an entourage including his personal physician Alfredo Boccia. The Paraguayan leader was brought in in a wheelchair.
Neither his spokesman nor the hospital had any immediate comment in Brazil.
A spokesman earlier told AFP however that Lugo was heading to Sao Paulo for treatment of "a formation in his neck related to his cancer... and how low his defenses are."
The spokesman insisted it was not a cancerous lump, and said Lugo -- whose face appeared reddish and had a visible swelling in his neck -- would likely remain at the hospital until the weekend.
Lugo was hospitalized late Thursday for blood tests and an X-ray because of a reaction to a local anaesthetic that was injected during a dental procedure.
Since his illness was revealed on August 6, Lugo has undergone three of six chemotherapy sessions, which have already left an effect. He has lost his hair and on his rare public appearances wears a cap to cover his baldness.
Lugo's health problems have increased the political uncertainty in recent weeks, and he now governs with a minority government after his principal ally, the Liberal Party, defected.
Last month his political opponents called on Lugo to hand over power to Vice President Federico Franco, a leader of the Liberal Party, after he was forced to postpone or cancel several foreign trips.
Lugo's election in 2008 raised hopes after 62 years of rule by the conservative Colorado party, including a dictatorship from 1954 to 1989. But Lugo's approval rating has slid from 90 percent after the election to 50 percent today, according to polls.
Source: AFP Global Edition