Aquino, 75, was swept into power by the peaceful uprising that ousted late dictator Ferdinand Marcos, cementing her as an icon of democracy.
Usually dressed in her trademark yellow in public, she has remained active in social and political causes. Most recently, she has been attending rallies calling for the resignation of President Gloria Macapagal Arroyo.
Kris Aquino, her voice breaking as she fought back tears, read a statement on live television that said her mother had gone in for tests after suffering from high blood pressure, difficulty breathing and fever during the Christmas and New Year holidays, then a persistent cough, loss of appetite and weight loss.
"The result showed our mother is suffering from cancer of the colon," she said.
She said her family knows that its affairs are "part of our country's history," but asked that her mother "be accorded her privacy."
"We ask you for your compassion and prayers for our mom's recovery," she said.
The presidential palace was saddened by the announcement and hopes for Aquino's speedy recovery, Arroyo spokesman Ignacio Bunye said.
A former housewife and political neophyte, Aquino reluctantly took over as Marcos' main challenger after her husband, opposition leader Benigno "Ninoy" Aquino Jr., was gunned down at Manila's airport upon returning from exile in 1983.
"I don't know anything about the presidency," the mother of five said in mid-1985, laughing off a seemingly ridiculous suggestion she run for president.
It turned out to be far from ridiculous. After a fierce campaign, voting was held Feb. 7, 1986, and journalists, foreign observers and church figures raised charges of massive fraud by Marcos.
The Commission on Elections ruled Marcos won by a slight majority; Aquino's camp figured she lost 25 percent of her votes through fraud.
The military brass mutinied, and hundreds of thousands of demonstrators, accusing Marcos of cheating, swept Aquino to power on Feb. 25 in a peaceful protest that became a harbinger of change in authoritarian regimes worldwide.
Marcos fled the country and died in Hawaii in 1989. Aquino held office until 1992, surviving at least six coup attempts.
"During Ninoy's incarceration and before my presidency, I used to ask why it had always to be us to make the sacrifice," she told a newspaper last year. "And then, when Ninoy died, I would say, why does it have to be me now? It seemed like we were always the sacrificial lamb."
Aquino promised that her administration would mark a complete change from the pomp and ostentatious displays of wealth during the Marcos years.
She moved her office to a whitewashed guest house in the palace complex and turned the ornate building where the Marcoses had lived into a museum.
Adopting a policy of national reconciliation, she freed more than 500 political prisoners, including some of the top leaders the Communist Party of the Philippines.
She oversaw the writing of a new constitution, but critics claimed that she fell short of the promise of social and economic reforms, which many of her supporters hoped would follow the ouster of Marcos.
Aquino again became active in 2001, throwing her support behind Arroyo, who was swept to power in the country's second "people power" revolt, toppling Joseph Estrada.
However, Aquino later took on Arroyo, joining opposition figures in calling for her resignation over allegations of vote-rigging in the 2004 elections and, more recently, of corruption.