It has been shown that mitral valve repair is superior to mechanical replacement in children with rheumatic heart disease, however there is limited data on the factors affecting the long-term durability of repaired mitral valves. We present here our experience following rheumatic mitral valve repair, adapting a risk score to clarify some novel predictors of repair instability. 79 children (median age 11.4 years) with rheumatic heart disease underwent their first mitral surgery between 1997 and 2015. Patients with concomitant aortic and tricuspid repair were included. Mean follow-up time was 7.72 years. Mitral deterioration (defined as cardiac death, reoperation or recurrent moderate to severe valvular disease) was used as a key endpoint. Preoperative echocardiographs were analysed and graded with respect to valvular mobility, subvalvular apparatus alteration and function. All patients had successful mitral valve repair. Seven patients died during follow-up. Kaplan-Meier analysis demonstrated survival at 15 years as 83%. 38 patients were deemed to have deterioration in mitral valve function, with freedom from deterioration at 15 years being 28%. Presence of an immobile anterior mitral leaflet preoperatively and the technique of posterior patch extension were shown to be significant determinants of mitral valve deterioration. The outcomes following mitral repair for rheumatic heart disease in the young are in keeping with previous studies. Whilst the survival is high, long-term valve stability was poor. Our findings suggest that the preoperative finding of a restricted anterior mitral leaflet is a negative predictor of repair durability.