Objective. To describe the quality assurance activities related to follow-up evaluation of abnormal screening mammograms and subsequent initial treatment of women determined to have breast cancer for the screening programs represented in the International Breast Cancer Screening Network (IBSN). Design. Analysis of data from a survey that included questions about screening program policies, standards, and procedures for follow-up of women with abnormal mammograms, as well as the data and measures that programs use to assess the adequacy of follow-up and initial treatment. Setting and participants. IBSN representatives in 23 countries completed a comprehensive questionnaire between May and December 1998. Results. Two-thirds of IBSN countries reported that they have a written policy or guidelines for follow-up of an abnormal mammogram; 64% require accreditation of the cytology or pathology laboratories that analyze breast specimens, or subject pathology laboratories to external audits. Of the 22 activities and measures related to quality of follow-up and initial treatment that we examined, all countries had in place at least half of them, although countries were more likely to have implemented activities and measures related to data collection and evaluation than to processes of care. Conclusions. Population-based screening mammography programs cannot achieve the goal of reducing breast cancer mortality if women with abnormal mammograms do not receive appropriate, timely follow-up and initial treatment. This study shows that IBSN countries vary in their implementation of procedures and measures to assure the quality of follow-up and initial treatment for women with abnormal screening mammograms. There is more emphasis on collecting and evaluating data than establishing mechanisms to ensure that the processes of care for follow-up and initial treatment are of high quality.