The first time Zellweger noticed that there was something wrong was when she examined her breasts on her own in 1996, and found two small masses in her right breast.
The Bridget Jones's Dairy star, who had lost her grandmother to cancer in 1969, decided to get them checked up.
"I thought, 'Oh, I know what you do now. You find out what that is'. So I went and found out," Contactmusic quoted her as telling America's Self magazine.
Zellweger revealed that she made an appointment with her doctor in Los Angeles, and went to his clinic with a no-nonsense attitude.
"I was like, 'Let's just have it over with. Get in there, stab away, get your little crunchy bit or whatever you're looking for, put it in the mail and we'll call it a day'.
"Then I had a weird couple of days waiting for the phone call," she recalled.
When the test results came, it was discovered that the actress had fibroadenomas (benign breast tumours), which she calls, "a fancy word for nothing".
But even though she has nothing to fear from the lumps in her breast, she has decided to take cancer threats more seriously.
"I pay attention, I do go get my check-ups," she added.