Prostheses used to treat heart valve disease improve patient survival, but have certain disadvantages. Paravalvular leakage (PVL) is a rare complication after mitral valve replacement (MVR), and can impair cardiac function and reduce the patient’s functional capacity, depending on the degree of periprosthetic regurgitation. Between 1985 and July 1999, 2,502 patients underwent MVR with or without concomitant cardiac procedures. Of these patients, 33 (18 males, 15 females; mean age 39.8+/-15.3 years; range: 12-62 years) had PVL of differing degree. The interval between MVR and observation of PVL was 30.5+/-31.5 months (range: 1-126 months), and the period after diagnosis was 22.6+/-31.5 months (range: 2-114 months). Fourteen patients (42.4%) underwent reoperation (RO group), and 19 (57.6%) were followed medically (ME group). Indications for reoperation were reduction of functional capacity, echocardiographically proven serious mitral regurgitation, and hemolysis. Reoperative mortality was 3.0% (1/33), and late mortality 3.1% (1/32) for all patients. Cumulative survival after PVL was 90.2+/-6.7% at both five and ten years. Annular calcification (33.0%) and infective endocarditis (18.2%) were important predictive factors for development of PVL. Only one patient required second re-do surgery. Univariate and forward stepwise logistic regression analyses showed that there was no predictor for the development of severe PVL requiring a second reoperation. No difference was observed between left ventricular dimensions before and after periprosthetic regurgitation. The only significant finding between groups was an increase in left atrial diameter in RO patients after the development of PVL (p <0.05). Among patients undergoing MVR there are no clinical features to distinguish who will develop severe PVL during follow up. If PVL reduces the patient’s functional capacity or causes serious hemolysis, or if severe PVL is evaluated echocardiographically, then reoperation must be performed. Mild or moderate mitral regurgitation without impairment of functional capacity may be followed medically. In asymptomatic patients, enlargement (>5%) of the left atrial diameter following development of moderate PVL may be a valuable criterion for deciding when to reoperate.