Spokesman David Ginsberg said Mrs. Edwards, 55, discovered a lump in her right breast while on a campaign trip last week. Her family doctor told her Friday that it appeared to be cancerous and advised her to see a specialist when she could.
She put off the appointment until Wednesday so as not to miss campaign time.
The Edwards family went straight to Massachusetts General Hospital from Boston's Faneuil Hall after Kerry and Edwards conceded on Wednesday.
Mrs. Edwards had a needle biopsy performed at the hospital, where Dr. Barbara Smith confirmed the cancer, Ginsberg said.
He said the cancer was diagnosed as invasive ductal cancer. That is the most common type of breast cancer, and can spread from the milk ducts to other parts of the breast or beyond.
Doctors say at age 55 she's in the highest risk group, reports CBS News Correspondent Elizabeth Kaledin.
"About two-thirds of women who get breast cancer are over the age of 50. So cancer is really a disease of aging, Dr. Deborah Axelrod told Kaledin.
Mrs. Edwards prognosis depends entirely on the size of the tumor and whether or not the cancer has spread to her lymph nodes. Treatment generally involves some combination of surgery, chemotherapy and radiation:
Thursday night, the Edwards were in Washington, D.C. where they are awaiting further test results that will dictate the course of her treatment. But aides say both Mrs. Edwards and her doctors are optimistic that this breast cancer is beatable, reports Kaledin.
Ginsberg said spirits are high at the Edwards household. "Everybody feels good about it, that this is beatable," he said.
Edwards, who leaves his North Carolina Senate seat in January, said in a statement, "Elizabeth is as strong a person as I've ever known. Together, our family will beat this."
The American Cancer Society estimated that nearly 216,000 American women will be diagnosed with breast cancer this year.
Treatments have been getting better. The current five-year survival rate for breast cancer is 87 percent, up from 78 percent in the mid-1980s. About 40,000 women die of breast cancer annually.
Overall, the society says about one in seven women will develop breast cancer in their lifetime.
Invasive ductal cancer accounts for 65 percent to 80 percent of all breast cancers, according to the Merck Manual of Medical Information.
Treatment usually begins with surgery, according to the National Cancer Institute. This could involve removal of the cancer itself and usually nearby lymph nodes. Lumpectomy, just removing the cancerous mass, is becoming more common, though sometimes removal of the whole breast is done.
Surgery can be followed by chemotherapy, radiation or hormone therapy.
Radiation can focus on a cancer site from a machine outside the body or use a radioactive substance placed near the cancer in "seeds" or via needle.
Chemotherapy uses drugs that can stop or slow the growth of cancer that may have spread.
Hormone therapy removes or blocks hormones that can encourage growth of cancer cells.
In early stages of cancer a combination of the drug tamoxifen and hormone therapy is commonly used, the Cancer Institute reports.
The Edwardses married in 1977. They have two daughters, Cate and Emma Claire and a son, Jack. Son Wade died in a 1996 traffic accident.
Mrs. Edwards, born in Jacksonville, Fla., grew up hopscotching between the United States and Japan. She met her future husband at University of North Carolina law school.
She juggled a successful legal career and family for 19 years. Then — stunned by Wade's death — she quit work to have more children at an age when many contemporaries were easing toward grandmotherhood.
On the campaign, she dubbed herself the "anti-Barbie," a quick-witted, down-to-earth political wife who connected particularly well with mothers and fathers.