Incidence of breast cancer in Norway and Sweden during introduction of nationwide screening: prospective cohort study.

To determine whether any increase in the incidence of breast cancer in women detected by mammography is compensated for by a drop in the incidence after age 69, years when women are no longer invited for screening. Population based cohort study of incidence of breast cancer during the introduction of nationwide screening programmes. Norway and Sweden. All women aged above 30 years (1.4 and 2.9 million, respectively, in 2000). Changes in age specific incidence rates of invasive breast cancer associated with the introduction of the screening programmes. As a result of screening the recorded incidence of breast cancer in women aged 50-69 years increased by 54% in Norway and 45% in Sweden. There was no corresponding decline in incidence after the age of 69 years. Without screening one third of all invasive breast cancers in the age group 50-69 years would not have been detected in the patients' lifetime. This level of overdiagnosis is larger than previously reported.