Background: Poly(ADP-ribose)polymerase (PARP) inhibitors have shown substantial activity in homologous recombination (HR) deficient ovarian cancer and are undergoing testing in other HR-deficient tumors. For reasons that are poorly understood, not all patients with HRD cancers respond to these agents. Preclinical studies have demonstrated that changes in alternative DNA repair pathways affect PARP inhibitor (PARPi) sensitivity. As this has not previously been assessed in the clinical setting, we examined the relationship between HRD score, BRCA1 and BRCA2 mutation status, expression of NHEJ pathway repair proteins, and response of ovarian cancers treated with single agent PARPi. Methods: Archival biopsies from ovarian cancer patients undergoing treatment on a single-agent PARPi trial were stained for PARP1, RAD51, and multiple components of the nonhomologous end-joining (NHEJ) pathway, including 53BP1, KU70, KU80 and DNA-PK_CS. Assays were validated by showing that the IHC signal was markedly attenuated with gene knockout or highly effective siRNA. Histochemistry- (H-) scores were determined for each repair protein in each sample. HRD score was determined from tumor DNA. Results: Responses to the PARPi ABT-767 were observed exclusively in ovarian cancers with an HR-deficiency. In this HR-deficient subset, 7 of 18 patients (39%) had objective responses, however actual HRD score did not further correlate with change from baseline tumor volume (r = 0.0499; p = 0.87). Conversely, in the HR-deficient subset, 53BP1 H-score showed a strong correlation with change from baseline tumor volume (r = -0.69, p = 0.004), followed by KU80 (r = -0.46; p = 0.08). Conclusions: Differences in complementary repair pathways, particularly in the NHEJ pathway, correlate with PARPi response of HR-deficient ovarian carcinomas. Among HR-deficient ovarian carcinomas, 53BP1 H-score demonstrated a strong correlation with the percent change of tumor volume.