Background: Breast cancer can be classified into molecular subtypes that have distinct survival patterns. The purpose of this study was i) to evaluate the prognostic significance of breast cancer subtypes in a cohort of women taking part in the NEAT and BR9601 clinical trials comparing CMF with ECMF, and ii) to evaluate whether the subtypes were predictive of the added benefit of epirubicin in these trials. Methods: Tumor tissue microarrays were stained and scored for ER, PR, HER2, EGFR and CK5/6. These were used to classify the tumors into six intrinsic subtypes (1). We used Cox regression to compare overall survival (OS), breast cancer specific survival (BSS) and relapse free survival (RFS) in the different subgroups. We also compared the effect of ECMF with CMF by subgroup. Results: IHC data were available for 1725 cases of whom 805 were Luminal 1-basal negative, 153 were Luminal 1-basal positive, 174 were Luminal 2, 192 were HER2-like, 230 were core basal phenotype and 171 were 5-negative phenotype. Median follow-up time was 7 years. The prognostic effects of the subtypes were similar to those reported for unselected breast cancer cases irrespective of adjuvant therapy (Blows FM, et al. PLoS Med 2010;75:e1000279.). In particular, the luminal 1-basal negative tumors were associated with the best prognosis in five years after surgery and the HER2-like tumors were associated with the poorest prognosis. ECMF has previously shown to be associated with a 33% relative risk reduction for OS compared to CMF (Poole CJ et al. N Engl J Med 2006;35518:1851-62.). There was little evidence for significant heterogeneity of effect by tumor subtype for any end point (OS P= 0.40, BSS P=0.53 RFS P=0.50). However, there was an observed trend towards the largest additional benefit from ECMF being in women with tumors of the 5-negative phenotype (OS HR=0.39 95% CI 0.21-0.73) and the smallest being in Luminal 1-basal negative tumors (OS HR=0.86 95% CI 0.64-1.16). Conclusions: In a clinical trial in which all patients received chemotherapy, we confirmed that breast cancer subtypes show distinct behaviour with differences in short and long term survival. The benefit of ECMF over CMF was statistically similar in all disease subtypes.