We report on our experience of quality of life (QL) assessment in adjuvant clinical trials of the International Breast Cancer Study Group (IBCSG), with special emphasis on cultural and logistical aspects of international organization that are unique to this group. Data are presented regarding submission rates of assessments before and after treatment failure, and timing of assessments relative to chemotherapy administration. To identify areas where rates might be improved, we investigated the association between missing data and sociodemographic and biomedical factors, treatment assignment, institution, chemotherapy compliance and toxicity in a trial of adjuvant chemoendocrine therapy for post-menopausal patients with breast cancer (IBCSG VII). The factors most highly associated with missing data were institution and chemotherapy compliance.