Background- To evaluate the previously reported excess of thromboembolic events during the 30 days after the end of study (EOS) visit when participants transitioned from blinded therapy to open-label vitamin K antagonist. Methods and Results- At the EOS visit, open-label vitamin K antagonist was recommended, and the international normalized ratio (INR) was not to be measured until 3 days later to preserve blinding. We analyzed transition strategies, clinical outcomes, and INR values. Event rates are per 100 patient-years. A total of 9248 (65%) participants were taking study drug at EOS, and, between days 3 and 30, an excess of stroke and systemic embolic events were observed in participants assigned to rivaroxaban (rivaroxaban 22 events, event rate 6.42; warfarin 6 events, event rate 1.73; hazard ratio, 3.72; 95% confidence interval, 1.51-9.16; P=0.0044). No INR values were reported for ≈5% of participants transitioned to warfarin. By 30 days after EOS, 83% of the warfarin group and 52% of the rivaroxaban group had ≥1 therapeutic INR value. Median time to first therapeutic INR was 3 days in the warfarin group and 13 days in the rivaroxaban group. Conclusions- The excess of events at EOS was likely because of a period of inadequate anticoagulation in rivaroxaban participants switched to vitamin K antagonist therapy. If transition from rivaroxaban to vitamin K antagonist is needed, timely monitoring and careful dosing should be used to ensure consistent and adequate anticoagulation.