Dr. Rueben Devlin from Humber Regional Hospital said Wednesday night that Ford had been complaining of abdominal pains and that an examination resulted in a working diagnosis of a tumor.
He said Ford had been suffering from stomach pains for at least three months and they had worsened in the last 24 hours.
"It became unbearable for him," he said.
Ford, 45, acknowledged using crack following months of denials. The mayor returned to work in June after a rehab stint for drug and alcohol abuse and is running for re-election on Oct. 27.
Devlin said doctors need to determine what type of tumor it is by doing a biopsy. He said he could not say how long Ford will be in the hospital.
Devlin said a CT scan done on Ford is "very definitive for the tumor" but a "definitive diagnosis" is still to come.
Ford had previously said he had abdominal surgery to remove a tumor on his appendix. "I had a major surgery on my appendix; they took out a piece of my colon. So I had a tumor in my appendix, and that's pretty scary when that happens," he told CTV television in a 2010 interview.
Doug Ford Sr., the mayor's father, died of colon cancer in 2006.
Councilor Doug Ford, the mayor's brother and campaign manager, said Rob is in good spirits.
Doug Ford sidestepped questions about the mayoral race, saying he would speak about the campaign on Thursday. Although campaigning has been under way for months, the official deadline for candidates to sign up is Friday. That leaves time for Doug Ford, a city councilor, to step in for his brother, a possibility some have long speculated about.
Doug deflected a question about Rob's previous health issues.
Doug said Rob complained of stomach pain while he had breakfast with Rob.
"He said his stomach was bothering him. He went to the doctor's and the doctor sent him over to Humber," Doug Ford said.
Dennis Morris, Ford's lawyer and a family friend, said he spoke to the mayor on Tuesday and he didn't mention any abdominal pain. Ford appeared well during a debate on Tuesday night.
The mayor has steadfastly refused to step down since reports emerged of a video of him smoking crack.
Olivia Chow, who is running to replace Ford, Tweeted that her thoughts and hopes are with Ford and his family for good news in the days ahead. John Tory, considered the front-runner in the race, wished Ford a speedy recovery.
"My thoughts are with Mayor Ford, Renata, their children and the entire Ford family this evening," Tory said in a statement.