The clinical and complete pathological response of a primary breast cancer to chemotherapy has been shown to be an important prognostic for survival. However, the majority of patients do not experience a complete pathological response to primary chemotherapy and the significance of lesser degrees of histological response is uncertain and the prognostic significance is unknown. The purpose of this study was to evaluate a new histological grading system to assess response of breast cancers to primary chemotherapy and to determine if such a system has prognostic value.A consecutive series of 176 patients with large (> or =4cm) and locally advanced breast cancers were treated with multimodality therapy comprising primary chemotherapy, surgery, radiotherapy and tamoxifen. All underwent assessment of the primary breast tumour before and after completion of chemotherapy. Residual tumour was excised after completion of chemotherapy (mastectomy or wide local excision with axillary surgery). The removed tissue was assessed and response to chemotherapy graded using a five-point histological grading system based with the fundamental feature being a reduction in tumour cellularity; comparison being made with a pre-treatment core biopsy. All patients were followed up for 5 years or more. Pathological responses were compared to 5 year overall survival and disease-free survival using log rank tests. The overall 5-year survival for all patients was 71%, and 5 year disease free interval was 60%. There was a significant correlation between pathological response using this new grading system and both overall survival (P=0.02) and disease-free interval (P=0.04). In a multivariate analysis of known prognostic factors, the Miller/Payne grading system was an independent predictor of overall patient survival. This grading system, which assesses the histological response to primary chemotherapy, can predict overall survival and disease-free interval in patients with large and locally advanced breast cancers treated with such therapy. The relationship of degree of histological response to overall and disease-free survival has been shown in univariate and multivariate analyses and could potentially have an important role in the clinical management of patients with locally advanced breast cancer undergoing primary chemotherapy.