Reply to the letter to the editor ''No major impact of mammography screening on the age specific incidence rates of breast cancer in the Netherlands'' Dear Sir, Implementation of mammographic screening programs for breast cancer has coincided with large changes in age specific incidence for this cancer in many countries. We agree with Verkooijen et al. that the reasons for such changes need to be established. 1 Shortly after we published our comparisons of age-incidence changes for breast cancer in some European countries, 2,3 an age-period-cohort analysis of breast cancer incidence trends was published for all the Nordic countries, 4 concluding ''that screening programmes have at least had three distinct effects on breast cancer incidence, namely, a large initial increase in rates following the initial screening round; a persistent, slightly higher risk while the women remain in the screening programme; and finally, a marked decrease in the rates when the women leave the program''. The study found a strong cohort effect in the incidence of breast cancer, and increases were noted even before the screening programs were implemented. However, referring to the literature, the Nordic authors concluded that only a small proportion of the increase is explained by known demographic and life-style factors, such as hormone displacement therapy.