OBJECTIVES: In recent years observational epidemiological studies have been used to estimate overdiagnosis in breast cancer screening. These estimates vary widely. In this paper we present some of the methodological issues which explain the large variability of the reported findings. METHODS: Different types of observational studies were identified according to study design, definition of the population, adjustment for breast cancer risk and adjustment for lead time. RESULTS: The majority of observational studies that have estimated breast cancer overdiagnosis have analyzed temporal trends or geographical differences in breast cancer incidence. Estimates of overdiagnosis in a dynamic population vary widely, from 4% to 52%. Only a few studies have used the cohort approach and they found estimates varying from 1% to 5%. CONCLUSIONS: The cohort approach is preferable to the analysis of a dynamic population because it allows the follow-up of a group of women who have had the opportunity for screening and evaluates if there is sufficient follow-up after the last screen.
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