The document outlines a system proposed by the International Union Against Cancer (UICC) for assessing the response to treatment in advanced breast cancer. The project, initiated in 1975, aimed to develop a uniform evaluation method to facilitate accurate comparisons of published results from different centers. Advanced breast cancer was chosen as the first tumor type to address due to the existing variety of evaluation methods. The final document, accepted by the British Breast Group, replaces their previous assessment method. The guidelines are intended for designing clinical trials to assess the objective response of locally advanced or metastatic breast cancer to treatment, excluding subjective response. Key criteria for eligibility include histological evidence of breast cancer, objective evidence of disease progression, and evaluable clinical disease. Exclusions consider factors such as debility, associated medical conditions, and previous antitumour therapy. The document details baseline studies, including history, performance, physical examination, photographs, radiographs, and laboratory studies. It also provides specific methods for measuring and recording lesions at various anatomical sites. Stratification is recommended to balance groups in controlled trials, considering factors like menopausal status, disease-free interval, and other prognostic variables. Follow-up studies are recommended every 4 weeks, with baseline studies repeated every 6 months unless symptoms develop. Response definitions include complete and partial regression, no change, and progressive disease. The duration of response is dated from the start of therapy until new lesions appear or existing lesions increase by 25% or more. Survival data should be recorded, and extra-mural review is recommended to ensure accuracy. The document acknowledges contributions from various experts and was developed during a meeting in Geneva in 1975, supported by the Ontario Cancer Treatment and Research Foundation.The document outlines a system proposed by the International Union Against Cancer (UICC) for assessing the response to treatment in advanced breast cancer. The project, initiated in 1975, aimed to develop a uniform evaluation method to facilitate accurate comparisons of published results from different centers. Advanced breast cancer was chosen as the first tumor type to address due to the existing variety of evaluation methods. The final document, accepted by the British Breast Group, replaces their previous assessment method. The guidelines are intended for designing clinical trials to assess the objective response of locally advanced or metastatic breast cancer to treatment, excluding subjective response. Key criteria for eligibility include histological evidence of breast cancer, objective evidence of disease progression, and evaluable clinical disease. Exclusions consider factors such as debility, associated medical conditions, and previous antitumour therapy. The document details baseline studies, including history, performance, physical examination, photographs, radiographs, and laboratory studies. It also provides specific methods for measuring and recording lesions at various anatomical sites. Stratification is recommended to balance groups in controlled trials, considering factors like menopausal status, disease-free interval, and other prognostic variables. Follow-up studies are recommended every 4 weeks, with baseline studies repeated every 6 months unless symptoms develop. Response definitions include complete and partial regression, no change, and progressive disease. The duration of response is dated from the start of therapy until new lesions appear or existing lesions increase by 25% or more. Survival data should be recorded, and extra-mural review is recommended to ensure accuracy. The document acknowledges contributions from various experts and was developed during a meeting in Geneva in 1975, supported by the Ontario Cancer Treatment and Research Foundation.