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BACKGROUND It remains unclear which patients with metastatic germ cell tumours (mGCTs) need prophylactic anticoagulation to prevent venous thromboembolic events (VTEs). OBJECTIVE To assess the risk and onset of VTEs stratified by risk factors. DESIGN, SETTING, AND PARTICIPANTS This multi-institutional retrospective dataset included mGCT patients treated with first-line platinum-based chemotherapy. INTERVENTION Patients with prophylactic anticoagulation were excluded. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS A regression analysis was performed to select risk factors for VTEs. The simulated number needed to treat (NNT) and the number needed to harm (NNH) with prophylactic anticoagulation were calculated based on the cumulative incidences retrieved from this study and hazard rates of recently published trials describing the efficacy of prophylactic anticoagulation to prevent VTEs and the risk of bleeding events. RESULTS AND LIMITATIONS From 1120 patients, 121 (11%) had a VTE, which occurred prior to chemotherapy in 49 (4%) and on or after chemotherapy in 72 (6%). Six patients (<1%) had a bleeding event without anticoagulation. After backward regression, the one risk factor for a VTE during or after chemotherapy was the use of a venous access device. The simulated cumulative VTE incidence from prophylactic anticoagulation for patients on or after chemotherapy would translate into an NNT of 45 (95% confidence interval [CI] 36-56) and an NNH of 186 (95% CI 87-506). Limitations are mainly related to the retrospective nature of the study. CONCLUSIONS The mGCTs associated VTEs are most common before and during, but not after, chemotherapy. Avoiding venous access device and/or prophylactic anticoagulation with an acceptable risk-benefit profile may decrease VTE occurring on chemotherapy. PATIENT SUMMARY We found that venous thromboembolic events (VTEs) occur rarely after chemotherapy. Based on experience of prophylactic anticoagulation in other cancers, we conclude that the risk of VTE in men undergoing chemotherapy for metastatic germ cell tumours can be decreased by thromboprophylaxis with a reasonable risk-benefit profile and by avoidance of venous access devices.