We report the case of a 41-year-old woman who developed thrombosis of a CarboMedics prosthetic valve in the mitral position despite adequate anticoagulation. This report provides further supportive data to recently published studies suggesting an increased thrombogenicity of the CarboMedics prosthetic valve in the mitral position and it underscores the importance of conducting large prospective, randomized trials for further evaluation of the thromboembolic complications of this valve.