The International Breast Cancer Study Group (formerly the Ludwig Group) has conducted nine clinical trials since 1978 (see the Appendix for participants and authors). Biologic hypotheses related to the combined use of chemotherapy and endocrine therapy in women with operable breast cancer were tested. Questions of timing of chemotherapy with respect to tumor surgery and late introduction of chemotherapy were also evaluated. Ongoing and future trials continue in this tradition to investigate combinations of available endocrine therapies and cytotoxic agents.