This study reports on the assessment of the vibrational serviceability performance of a parallel cable-stayed bridge, which was subjected to a vortex-induced vibration (VIV) in 2011, by identifying modal damping ratios from the operational monitoring data that were obtained. The natural excitation technique (NExT) combined with the eigen realization algorithm (ERA) was applied for the output-only modal analysis. Parameters regarding the NExT and ERA procedures were determined from a sensitivity analysis of identified damping ratios. Because a multiple tuned mass damper (MTMD) was installed at the center of the main span to mitigate VIV, the modal damping ratios before and after the installation of the MTMD were also compared. Several sets of operational monitoring data that had been collected under various windy conditions were used to develop a relationship between the identified damping ratios and the corresponding VIV level of the bridge. On the basis of these findings, the performance of the bridge was enhanced regarding vibrational serviceability. (C) 2016 American Society of Civil Engineers.