Background: The Norwegian Breast Cancer Screening Program (NBCSP) was implemented across the country in 2005 and has been criticised for potential 'overdiagnosis', i.e. a breast cancer diagnosis that otherwise would not have been detected or treated in a woman's lifetime. We aimed to estimate overdiagnosis in the NBCSP based on the Norwegian Women and Cancer (NOWAC) study using both questionnaire information and record linkage information from NBCSP. Method: For 124,978 women aged 49-79 years from the NOWAC study, information on screened women could be cross-validated from the NBCSP database. Based on information from the NOWAC questionnaire, unscreened women were further divided into those who had mammograms taken only outside the NBCSP and those who had never had taken a mammogram. Breast cancers diagnosed in 2005-2013 were identified through linkage to the Cancer Registry of Norway; in situ or DCIS 417; invasive 2845; combined 3262. Cumulative incidence rates (CIRs) for ages 49-79 years of breast cancer were compared using the log-rank test. Results: After exclusion of women with a family history of breast cancer, screened women had a CIR of 9.7% for combined breast cancer, non-significantly lower compared with unscreened women. Screened women had a 1.1% increased CIR or 13.0% increased relative risk of breast cancer diagnosis (significant) compared with women who had never had a mammogram, but for invasive breast cancer alone the difference was reduced to -0.2% (95% CI: -9.1; 8.8). Invasive breast cancers were significantly smaller (<2.5 cm) in screened versus unscreened women. There was a borderline significant decrease in lymph node positive cancer among screened (p = 0.06). Conclusion: The findings of no significant overdiagnosis combined with smaller tumours and less lymph node metastases suggest that the prevailing view of overdiagnosis in the NBCSP should be challenged.