Additional Contributions: The directors of the Anticoagulation Clinics of Amersfoort (Mark H. H. Kramer, MD), Amsterdam (Mary Remkes, MD), The Hague (Eric van Meegen, MD), Rotterdam (Harry A. A. H. Kasbergen, MD), and Utrecht (Hanneke de Vries-Goldschmeding, MD), all in the Netherlands, made the recruitment of patients possible. The interviewers (Jose C. M. van den Berg, Brigitte Berbee, Saskia van der Leden, Mieke Roosen, and Liesbeth C. Willems of Brilman) performed the blood draws. We also thank Ingeborg de Jonge, MSc; Rebecca Roelofsen, MSc; Marion Streevelaar; Lucie M. J. Timmers, MSc; and Ank J. Schreijer for their secretarial and administrative support and data management. The fellows Jeanet W. Blom, MD; Astrid van Hylckama Vlieg, PhD; Elisabeth R. Pomp, MSc; Lidwine W. Tick, MD; and Karlijn J. van Stralen, MSc, took part in every step of the data collection. Carla J. M. van Dijk, Rob van Eck, Jeroen van der Meijden, Petra J. Noordijk, and Thea Visser performed the laboratory measurements. Hans L. Vos, PhD, supervised the technical aspects of DNA analysis. We express our gratitude to all individuals who participated in the Multiple Environmental and Genetic Assessment.