Background Increased incidence of carcinoma in situ (CIS) has generally been attributed to widespread application of mammography screening. However, direct evidence for the impact of screening is rare.

Methods Screen detected and total CIS (ICD-10 D05) incidence data were obtained, between 1990 and 2007 where possible, from regional cancer registries in the UK and the Republic of Ireland (ROI). Age-standardised incidence rates for the UK and ROI, and regional screen detected incidence rates were analysed.

Results The total incidence rate of CIS among women aged 50–64 years increased by 3.1-fold in the UK (1990–2007) and by 2.9-fold in the ROI (1994–2007). An increase of 7.3-fold in the UK and 3.5-fold in the ROI was observed in women aged 65–69 years. Finally, an increase of 8.3-fold in the UK and threefold in the ROI occurred in the age group 70–74 years. Screen detected incidence rates for CIS largely parallelled the total incidence rates for CIS.

Conclusion Our results show increased incidence of CIS in all age groups investigated, with the greatest increase observed in women over 65 years. Our regional screen detected incidence rates indicate that increased incidence may partly be attributed to screening. However, other contributing factors must also be considered for the apparent increase in CIS incidence outside the screening age and the consistent difference observed between the screen detected rate and the total incidence rates for CIS.