Background: We have investigated whether the introduction of organized mammography screening in Norway led to less aggressive breast cancer surgery in the invited age group. Methods: We obtained aggregate incidence and surgical treatment data for women treated for ductal carcinoma in situ or invasive breast cancer from the Norwegian Cancer Registry. Rates of breast operations (mastectomy plus breast conserving therapy) and rates of mastectomy only for three age groups; women aged 40-49, 50-69 and 70-79 years were calculated. Changes in rates from a pre-screening period (1993-95) to a screening introduction phase (1996-2004) and then to a screening period (2005-08) are presented as hazard ratios comparing invited and non-invited women. Results: From the pre-screening period (1993-95) to the screening period (2005-08) the annual breast operation rate increased by 70% (HR=1.70 95% CI 1.62 to 1.78), from 180 to 305 per 100 000 women in the invited age group (50-69 years). In contrast, the increase was only 8% (HR=1.08 95% CI 1.00 to 1.16), from 133 to 144 per 100 000 women per year in the younger, non-invited age group (40-49 years) while an 8% decline (HR=0.92 95% CI 0.86 to 1.00), from 227 to 214 per 100 000 women per year occurred in the older, non-invited age group (70-79 years). In the screening period (2005-08) mastectomy rates fell similarly in invited and non-invited women. However, during the screening introduction phase (1996-2004) the mastectomy rate was 31% (HR=1.31 95% CI 1.20 to 1.43) higher in screened compared to non-screened younger age group. Annual mastectomy rates increased in invited from the pre-screening period to the screening introduction period from 156 to 167 per 100,000 women compared to a decline from 109 to 91 per 100,000 women in younger non-invited. Conclusions: Mammography service screening is associated with a marked increase in breast operation rates in the invited age group and also an increase in mastectomy rates. While overdiagnosis likely caused the initial increase in mastectomy rates and the overall increase in use of surgery in the age group subjected to screening, the more recent decline in mastectomy rates has affected all age groups and is likely to have been caused by changes in surgical policy.