In 1982, during a pro surf meet in Huntington Beach, California, Sunn felt a lump in her breast which turned out to be breast cancer. When she was diagnosed in 1983, her prognosis was for one year. Sunn continued to surf every day after her diagnosis, despite the pain and chemotherapy associated with the disease. Following her diagnosis, Sunn became a radio disc jockey and surf reporter, a physical therapist at a Waianae care home, and a counselor at a cancer research center. She helped pilot a program for breast cancer awareness at the Wai'anae Cancer Research Center that involved educating local women about the causes and prevention of breast cancer.
Over the next 14 years, her cancer went into remission three times, and she underwent a mastectomy and a bone marrow transplant.
Rell Sunn died on January 2, 1998, aged 47. She lived almost 16 years with cancer instead of given her a year at first. More than 3,000 people attended her memorial service, where her ashes were scattered in the ocean off her native Makaha.