We report the case of a 76-year-old lady who had a mitral valve (MV) replacement for severe MV stenosis. Several days following the procedure, a routine transthoracic echocardiogram (TTE) was performed. Despite the prosthesis leaflets opening well anterograde maximum velocities were elevated; MV peak velocity of 2.4 m/s. A transoesophageal echocardiogram (TEE) demonstrated the superior strut of the prosthesis appearing to rotate inwards such that the ventricular (non-fixed) end of the struts approximate to one another during systole, reducing the effective orifice area for anterograde flow. A 3D TEE was performed. This showed that the valve struts were not parallel, with the planes of the struts converging from the base to the apex. However, this abnormal conformation appeared to be fixed rather than worsening in systole. The apparent movement of the posterior strut seen in the 2D TEE images appears to have resulted from systole movement of this strut partially out of the plane of the TEE (2D) cut. We believe the case demonstrates the value of 3D real-time TEE in the assessment of the MV prosthesis function.