Objective: Oral anticoagulation with vitamin K antagonists is recommended after mechanical heart valve replacement. However, data regarding the association between the quality of vitamin K antagonist treatment and the risk of complications are sparse.Methods: Patients undergoing mechanical heart valve replacement (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) with available data on international normalized ratio values were identified in Danish registries. The quality of vitamin K antagonist treatment between discharge after valve replacement and 6 months postdischarge (ie, index) was assessed as time in therapeutic range (TTR) !70% or<70% reflecting the percentage of time in therapeutic international normalized ratio interval. Patients were followed from index until occurrence of an outcome of interest (ie, thromboembolism and bleeding), death, or end of study (December 31, 2012), whichever came first. The risk of outcomes according to quality of vitamin K antagonist treatment was estimated with multivariable Cox regression.See Commentaries on pages 84 and 86.More than 100 million people worldwide experience valve diseases and the prevalence is expected to increase concurrently with increasing life expectancy. 1 Worldwide, From the