The purpose of the Women's Health Initiative (WHI) is to address cardiovascular disease, cancers of the breast and colon/rectum, and osteoporosis, the most common causes of death, disability, and impaired quality of life in postmenopausal women. The major components of the WHI are: a randomized controlled clinical trial of postmenopausal Hormone Therapy (HT), Dietary Modification (DM), and Calcium/Vitamin D supplementation (CaD); and a companion observational study (OS). The Clinical Trials and Observational Study components were completed in 2005; the WHI Extension Study continues to follow all consenting participants for health outcomes and selected exposures. The purpose of this additional follow-up is to describe the longer term effects of the original interventions, to document change in hormone use in participants from the HT trials, to expand the range of scientific questions that can be reliably addressed in the WHI, and to provide an infrastructure able to support additional investigations requiring some of the unique resources of a very large longitudinal study of postmenopausal women.