Background: Therapeutic options for patients with advanced germ cell tumors (GCTs) after multiple relapses or resistant disease are limited. Olaparib is an inhibitor of poly ADP ribose polymerase (PARP), an enzyme involved in DNA repair. We aimed to evaluate olaparib activity in patients with refractory GCT. Methods: In this proof-of principle open-label, single-arm, phase II trial of olaparib 300 mg twice daily in patients with relapsed/refractory metastatic germ cell cancer IGG-02 study (NCT02533765), patient eligibility included failure after high-dose chemotherapy or after at least 2 different cisplatin-based regimens. Measurements of serum tumor markers and computed tomography were carried out at baseline and every 6 weeks of olaparib treatment. The study primary endpoint was the overall response rate, the study planned to recruit initially 18 patients and not continue further recruitment until one or more responses were observed. Results: Between September 2015 and February 2019, 18 patients, median age 39 years (range, 22-61) were enrolled. The number of prior chemotherapy regimens was: 2 for 3 patients (16.7%), 3 for 5 patients (27.8%), >3 for 10 patients (55.6%). Sixteen cases (89.9%) received prior high-dose chemotherapy with support of hematopoietic progenitor cells. Grade 3-4 adverse events were observed in 5 patients (27.7%). There were no partial responses, 5 cases (27.8%) with stable disease (SD) lasting 3, 4, 4, 7 and 7+ months and 13 (72.2%) progressive disease. The 12-week progression-free survival probability was 27.8% [95% confidence interval (CI): 10.1%-48.9%]. The 12-month overall survival probability was 27.8% (95% CI: 10.1%-48.9%). A germline DNA repair profile panel showed only a BRCA1 mutated case associated with a SD lasted 4 months. Conclusions: Olaparib as a single agent has marginal activity in heavily pretreated GCT patients, however, an anecdotic 4-month SD in the only BRCA mutated patient has been reported. Plans for future studies with olaparib are suggested in combination or following salvage chemotherapy in less pretreated and more selected GCT patients. The Study has been conducted with AstraZeneca contribution. Clinical trial information: NCT02533765.