HER2 gene amplification and topoisomerase IIα gene (TOP2A) alteration have been associated with increased benefit from anthracycline compared to non-anthracycline containing adjuvant breast cancer chemotherapy in some but not other studies. Chromosome 17 centromere (CEP17) duplication was measured on TMAs from formalin-fixed paraffin-embedded specimens obtained from 639 of 716 premenopausal women with node positive breast cancer who received cyclophosphamide, epirubicin and fluorouracil (CEF) or cyclophosphamide, methotrexate and fluorouracil (CMF) in the randomized controlled mammary 5 (MA.5) adjuvant trial. The prognostic impact of CEP17 duplication and its interactions with treatment were studied for relapse-free survival (RFS) and overall survival (OS). Overall, CEP17 duplication was not significantly associated with RFS or OS in multivariate analysis. For patients whose tumours had normal CEP17 copy number there were no apparent benefits for CEF compared to CMF for RFS (HR 0.98; 95% CI 0.68-1.42) or OS (HR 1.10; 95% CI 0.72-1.69). For patients whose tumours had CEP17 duplication, there was significant benefit for CEF compared to CMF for RFS (HR 0.54; CI 0.33-0.89) and a trend towards significance for OS (HR 0.64; CI 0.37-1.09). The adjusted P values for interaction between treatment and CEP17 duplication were 0.09 for RFS and 0.13 for OS. This study suggests that CEP17 duplication has a borderline association with clinical responsiveness to anthracycline containing chemotherapy similar to previous results seen with HER2 amplification and TOP2A alteration in MA.5. An appropriately powered meta-analysis is required to discriminate the predictive value of these three candidate markers.