Researchers at the Los Angeles BioMedical Research Institute and the Fred Hutchinson Cancer Research Center in Seattle found women who took HRT just as menopause was beginning had a 41 percent higher risk of breast cancer than those who waited.
Another study followed one million British women and showed that those who waited to begin taking HRT for five years or more from menopause onset had little or no increased risk of breast cancer, but those who began the therapy before or as they entered menopause had a 43 percent higher risk.
The risk pattern stayed the same in spite of different hormonal therapies used, different durations the therapy was taken and in different body sizes, from lean to obese, Dr. Valerie Beral of Britain's Oxford University and colleagues wrote.
The number of women opting for hormone replacement therapy has declined in recent years. More than 16 million women used HRT in 2001 verses 6 million in 2009.