Objective To assess the incidence of early stage and advanced stage breast cancer before and after the implementation of mass screening in women aged 70-75 years in the Netherlands in 1998. Design Prospective nationwide population based study. Setting National cancer registry, the Netherlands. Participants Patients aged 70-75 years with a diagnosis of invasive or ductal carcinoma in situ breast cancer between 1995 and 2011 (n=25 414). Incidence rates were calculated using population data from Statistics Netherlands. Main outcome measure Incidence rates of early stage (I, II, or ductal carcinoma in situ) and advanced stage (III and IV) breast cancer before and after implementation of screening. Hypotheses were formulated before data collection. Results The incidence of early stage tumours significantly increased after the extension for implementation of screening (248.7 cases per 100 000 women before screening up to 362.9 cases per 100 000 women after implementation of screening, incidence rate ratio 1.46, 95% confidence interval 1.40 to 1.52, P<0.001). However, the incidence of advanced stage breast cancers decreased to a far lesser extent (58.6 cases per 100 000 women before screening to 51.8 cases per 100 000 women after implementation of screening, incidence rate ratio 0.88, 0.81 to 0.97, P<0.001). Conclusions The extension of the upper age limit to 75 years has only led to a small decrease in incidence of advanced stage breast cancer, while that of early stage tumours has strongly increased.