We analyzed the relation between age at diagnosis and relative survival (ratio of observed to expected survival) in 57,068 women in Sweden in whom breast cancer was diagnosed in 1960 to 1978 (about 98 percent of all cases). Women who were 45 to 49 years old had the best prognosis, with a relative survival exceeding that of the youngest patients (less than 30 years) by 7.6 to 12.9 percent at different periods of observation. Relative survival declined markedly after the age of 49--particularly in women aged 50 to 59--and the oldest women (greater than 75) had the worst rate. The difference in relative survival between those older than 75 and those 45 to 49 increased from 8.6 percent at 2 years to 12.2, 20.3, and 27.5 percent after 5, 10, and 15 years of follow-up, respectively. The long-term annual mortality rate due to breast cancer approached 1 to 2 percent at the premenopausal ages but exceeded 5 percent throughout the period of observation in the oldest age group. An understanding of the biologic basis for the complex relation between age and prognosis might provide a better understanding of the natural history of breast cancer in women.