New research suggests that breast cancer can come back as many as 20 years after successful treatment.
British researchers at Oxford University studied more than 60,000 women for 20 years.
Every patient received pill treatments which block the effects of oestrogen or shut off the hormone's supply. After five years of therapy, their cancers had gone and they stopped taking the drugs.
The scientists found cancer that had been dormant was most likely to return in women with larger tumors and cancer that had spread to the lymph nodes.
The most common form of breast cancer recurred for 40 percent of women in that group.
Women with small, low-grade cancers and no spread to the lymph nodes had only a 10 percent chance of cancer recurrence.
Recent research has suggested that extending hormone therapy to 10 years may be more effective at preventing breast cancer recurrence and death.