Acknowledgment: We thank the directors of the Anticoagulation Clinics of Amersfoort (M. H. H. Kramer, MD), Amsterdam (M. Remkes, MD), Leiden (F. J. M. van der Meer, MD), The Hague (E. van Meegen, MD), Rotterdam (A. A. H. Kasbergen, MD), and Utrecht (J. de Vries-Goldschmeding, MD), who made the recruitment of patients possible. The interviewers (J. C. M. van den Berg, B. Berbee, S. van der Leden, M. Roosen, and E. C. Willems of Brilman) performed the blood draws. We also thank I. de Jonge, MSc, R. Roelofsen, MSc, M. Streevelaar, L. M. J. Timmers, MSc, and J. J. Schreijer for their secretarial and administrative support and data management. The fellows J. W. Blom, MD, A. van Hylckama Vlieg, PhD, E. R. Pomp, MSc, L. W. Tick, MD, and K. J. van Stralen, MSc, took part in every step of the data collection. C. J. M. van Dijk, R. van Eck, J. van der Meijden, P. J. Noordijk, and T. Visser performed the laboratory measurements. We also thank M. den Heijer, MD, for providing the individual study data from his meta-analysis. We express our gratitude to all individuals who participated in the MEGA Study.