Objective: 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.
Design: Population based cohort study of incidence of breast cancer during the introduction of nationwide screening programmes.
Setting: Norway and Sweden.
Participants: All women aged above 30 years (1.4 and 2.9 million, respectively, in 2000).
Main outcome measures: Changes in age specific incidence rates of invasive breast cancer associated with the introduction of the screening programmes.
Results: 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.
Conclusions: 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.