Background: Cisplatin based NAC prior to cystectomy is standard of care for MIBC, with 40-50% expected to respond with ≤pT1N0M0. Biomarkers predictive of response are lacking. Methods: MIBC pts who received 3 cycles of cisplatin based NAC on 1 of 2 prospective multicenter clinical trials were included. Pts treated with accelerated methotrexate, vinblastine, doxorubicin + cisplatin (AMVAC) provided the discovery set [n=34, 15/34 (44%) ≤pT1N0M0]. Pts treated with dose dense gemcitabine + cisplatin (DDGC) provided the validation set [n=24, 11/24 (46%) ≤pT1N0M0]. DNA from pre-treatment tumor tissue underwent sequencing for all coding exons of 287 cancer related genes and was analyzed for presence of base substitutions, indels, copy number alterations, and selected re-arrangements. The mean number of variants and variant status for each gene were correlated with response using two-sample t-test and Fisher’s exact tests. Variant data were used to create a classification tree to discriminate responders vs. non-responders in the AMVAC discovery cohort. The resulting decision rule was then tested in the independent DDGC validation set. Overall survival analysis was performed using Kaplan-Meier. Results: Pts with pT0 had significantly more alterations than those with residual tumor in both the AMVAC discovery (p=.024) and DDGC validation (p=0.018) set. In the AMVAC discovery set, alteration in ≥1 of the three DNA repair genes ATM, RB1 or FANCC predicted for ≤pT1N0M0 (p<0.001, 87% sensitivity, 100% specificity) and improved overall survival (OS) (p=0.007). This test remained predictive for ≤pT1N0M0 in the DDGC validation set (p=0.033), with a trend towards improved OS (p=0.07) at short median follow up of 14.3 mo. Conclusions: Alterations in ≥1 of ATM, RB1 and FANCC predict response to cisplatin based chemotherapy defined as ≤pT1N0M0 in both our AMVAC discovery and DDGC validation sets. We hypothesize that defects in these genes, which are important for maintenance of chromatin structure and DNA repair, confer sensitivity to DNA damaging chemotherapy and explain the accumulation of alterations seen among pts with pT0. External validation in collaboration with the cooperative groups is planned.