We present the case of a 42-year-old man who underwent tissue mitral valve replacement for symptomatic mitral stenosis. Post-operative course was unremarkable but three-dimensional transoesophageal echocardiography clearly indicated that one cusp had very restricted motion with incomplete opening and premature closure. The cause of this early single cusp failure is unclear. It was not related to flow effects. It is conceivable that cusp failure such as here might contribute towards the degeneration of tissue valve replacements.