Just months after becoming president, Francois Mitterrand was diagnosed with the prostate cancer that killed him, but he kept it secret for 10 years, a newspaper reported Tuesday. Despite promises of openness about his health, unprecedented for a French leader, Mitterrand and his doctors did not disclose the cancer until September 1992, when he had his first prostate operation, the daily Le Monde reported. Mitterrand died Monday at age 79. Joelle Jaillette, a spokeswoman at Mitterrand's office, would not confirm the report. In May 1981, just 10 days after Mitterrand defeated incumbent Valery Giscard d'Estaing, Mitterrand's private physician Claude Gubler vouched for his good health. The cancer, found in tests a few months later, was so advanced the doctors rejected surgery, Le Monde said. Mitterrand began taking medication, including hormone treatment, which kept the cancer in check for about 10 years, the report said. Mitterrand was probably unaware of his cancer when he vowed complete "transparency" in personal health matters right after taking office, it said. The promise was unprecedented. Giscard did not publish a single health bulletin during his seven years in office. Although George Pompidou was in obvious pain from lymphoma during his last days in office, officials denied his health problems. On Tuesday, the rain-splattered sidewalk by Mitterrand's Left Bank apartment was carpeted with single-stem red roses _ the symbol of the Socialist victory in 1981 _ laid by thousands of mourners. Many stood in silence outside the office near the Champs de Mars, as family members, friends and former Cabinet ministers filed in. The Socialist Party planned a rally this evening at the Place de la Bastille. On Thursday, a national day of mourning for Mitterrand, funeral services are at Notre Dame cathedral.