Résumé : Background: The purpose of this study was to evaluate the prognostic and predictive value of p27 expression in patients with early breast cancer. Patients and methods: Quantitative immunofluorescence assays for p27 were done on a tissue microarray that included 823 samples from patients randomized between anthracycline-based chemotherapy and no chemotherapy. Quantification of p27 was done using the AQUA® system (HistoRx, Inc., Branford, CT). Both p27 nuclear expression and the nuclear to cytoplasmic ratio were assessed. Results: Nuclear p27 expression was not predictive for the efficacy of anthracycline-based chemotherapy [adjusted P = 0.18 for disease-free survival (DFS)] nor prognostic [95% confidence interval (CI) 0.99-1.01, P = 0.49]. However, p27 nuclear/cytoplasmic ratio was predictive for the efficacy of adjuvant chemotherapy (adjusted P = 0.016 DFS). The adjusted hazard ratio (HR) for relapse associated with adjuvant chemotherapy was 0.56 (95% CI 0.37-0.84, P = 0.005) and 1.06 (95% CI 0.76-1.47, P = 0.74) for patients with high and low nuclear/cytoplasmic ratio, respectively. p27 N/C ratio was prognostic in patients treated with chemotherapy (HR for relapse or death for a 1 unit increase in p27 N/C ratio was 0.30, 95% CI 0.12-0.77) but not in the untreated arm (HR for relapse or death was 1.27, 95% CI 0.58-2.8). Conclusions: This study did not confirm the role of p27 nuclear expression as a prognostic parameter. However, the p27 nuclear/cytoplasmic ratio was predictive in patients treated with anthracycline-based chemotherapy. © The Author 2012. Published by Oxford University Press on behalf of the European Society for Medical Oncology. All rights reserved.