Tricuspid regurgitation (TR) in patients with mitral valve (MV) disease is associated with poor outcome and predicts poor survival, heart failure, and reduced functional capacity. It is common if left untreated after MV replacement mainly in rheumatic patients, but it is also common in patients with ischemic mitral regurgitation. It is less common, however, in those with degenerative mitral regurgitation. It might appear many years after surgery and might not resolve after correcting the MV lesion. Late TR might be caused by prosthetic valve dysfunction, left heart disease, right ventricular (RV) dysfunction and dilation, persistent pulmonary hypertension, chronic atrial fibrillation, or by organic (mainly rheumatic) tricuspid valve disease. Most commonly, late TR is functional and isolated, secondary to tricuspid annular dilation. Outcome of isolated tricuspid valve surgery is poor, because RV dysfunction has already occurred at that point in many patients. MV surgery or balloon valvotomy should be performed before RV dysfunction, severe TR, or advanced heart failure has occurred. Tricuspid annuloplasty with a ring should be performed at the initial MV surgery, and the tricuspid annulus diameter (>or=3.5 cm) is the best criterion for performing the annuloplasty. In this article we will review the current data available for understanding the prognostic implications, mechanism, and management of TR in patients with MV disease.