Doctors at the Sheba Medical Centre near Tel Aviv said Sharon, who had a stroke on January 4, had problems with the functioning of his kidneys and was amassing liquids in his body.
Sharon's family are with him, the hospital said. Some of his former aides said there was no immediate danger to his life.
"Over the past two days the doctors have identified a deterioration in the kidney function ... and changes in brain tissue," a statement issued by the hospital said.
"The doctors are continuing tests in order to diagnose the changes which have taken place in Sharon's body and to administer the appropriate treatment."
Sharon's deputy, Ehud Olmert, assumed his powers on a temporary basis following the stroke. After the March 28 general election he took over as prime minister.