Background: The ceiling-drug effect seen for most active drugs in STS could be related, partially, to multidrug resistance mechanisms (MDRM). We previously reported the independent prognostic role for RFS and OS of MRP1 in high-risk localized STS of limbs and trunk-wall treated with epirubicin and ifosfamide (Mol Cancer Ther.2014 13(1):249-59). A translational study was carried out within the randomized phase III trial of epirubicin plus ifosfamide vs histotype-tailored neoadjuvant chemotherapy (NCT01710176), to investigate MRP1 prognostic value using the trial population as validation set. Methods: Patients enrolled in the trial were invited to participate, through the informed consent, to this analysis. IHC used QCRL-1 (Santa Cruz biotechnology) MRP1 monoclonal antibody. TMAs were built on the highest-grade area of each tumor, being the procedure blinded for clinical data. MRP1 expression was grouped as low (≤ 25% positive cells) vs high ( > 25% positive cells) expression. For data analysis, patients were grouped as A) epirubicin plus ifosfamide control arm and B) histotype-tailored experimental arm. Drugs used in group B were: gemcitabine-docetaxel (UPS), gemcitabine-DTIC (LMS), trabectedin (High-grade (HG) myxoid LPS), ifosfamide-etoposide (MPNST) and high-dose ifosfamide (SS). Prognostic value of MRP1’s extension was analyzed using Cox’s proportional hazard regression. A p-value < 0.05 was considered statistically significant. Results: 175 patients were analyzed (median age 49; males 61%) with median follow-up of 4.66 y. Group A (n = 88) included HG-myxoid LPS (27%), SS (25%), UPS (24%), LMS (12%) MPNST (10%) and others (2%); group B (n = 87) included UPS (38%), SS (24%), HG-myxoid LPS (20%), LMS (10%) and MPNST (8%). MRP1 high extension was distributed as follows: 48% (A) and 57% (B). High MRP-1 expression showed significantly worse prognosis for disease-free survival (DFS) (HR 2.71 (1.31-5.62) p = 0.007) and a trend towards worse OS (HR = 2.75 (0.97-7.81) p = 0.058) in group A. No correlation was seen between MRP-1 expression and DFS (p = 0.384) or OS (p = 0.665), in group B. Conclusions: MRP1 overexpression was related to significant worse prognosis in 2 prospective randomized series of high-risk, localized, STS treated with neoadjuvant epirubicin and ifosfamide. These agents are both substrate of MRP1; this could add rationale for a possible predictive role, as MDRM, for the two most active drugs in STS. A trial combining epirubicin, ifosfamide and MRP1 inhibitor is currently under design.