Mortality forces in women with cancer of the breast were measured by calculating the rate of dying and determining the cause of death in women who develop breast cancer. In the Syracuse-Upstate Medical Center Cancer Registry, 1,513 patients were followed for 15 years. The death curve of this group assumed a major slope characteristic of a single exponential curve, with a half life of 5.9 years. The cause of death was examined in a randomly selected group of patients from the Ontario Cancer Foundation, Hamilton, Ontario, and a group from the Syracuse Registry dying after 10 years. In these 136 patients, 130 died of breast cancer. From the National Cancer Institute Cooperative Studies of 3,225 women undergoing treatment for primary breast cancer, 914 died during the study---705 of cancer of the breast and 209 of competing risks. These data suggest that 80-85% of all women who die after developing cancer of the breast die of their breast cancer. Modification of the time of dying (rate of dying) or cause of death should be used as objectives of management rather than 5-year survival figures.