Objective BRCA-positive ovarian cancer patients derive benefit PARP inhibitors. Approximately 50% of ovarian cancer tumors have homologous recombination (HR) deficiencies and are therefore "BRCA-like," possibly rendering them sensitive to PARP inhibition. However, no predictive assay exists to identify these patients. We sought to determine if irradiation-induced Rad51 foci formation, a known marker of HR, correlated to PARP inhibitor response in an ovarian cancer model. Methods Ovarian cancer cell lines were exposed to PARP-inhibitor ABT-888 to determine effect on growth. Rad51 protein expression prior to irradiation was determined via Western blot. Cultured cells and patient-derived xenograft tumors (PDX) were irradiated and probed for Rad51 foci. In vivo PDX tumors were treated with ABT-888 and carboplatin; these results were correlated with the ex vivo ionizing radiation assay. Results Three of seven cell lines were sensitive to ABT-888. Sensitive lines had the lowest Rad51 foci formation rate after irradiation, indicating functional HR deficiency. Approximately 50% of the PDX samples had decreased Rad51 foci formation. Total Rad51 protein levels were consistently low, suggesting that DNA damage induction is required to characterize HR status. The ex vivo IR assay accurately predicted which PDX models were sensitive to PARP inhibition in vitro and in vivo. ABT-888 alone reduced orthotopic tumor growth by 51% in A2780ip2 cell line, predicted to respond by the ex vivo assay. Three PDX models' response also correlated with the assay. Conclusions The ex vivo IR assay correlates with response to PARP inhibition. Analysis of total Rad51 protein is not a reliable substitute. © 2014 Elsevier Inc.