Abstract Background Leaflet escape is a rare complication of mechanical heart valves. The following case is the first report of leaflet escape from an On-X mechanical mitral valve in a child. Case summary A 12-year-old boy presented with acute pulmonary oedema one year after mitral valve replacement with an On-X valve for rheumatic heart disease. Transoesophageal echocardiogram diagnosed an absent mechanical mitral valve leaflet and severe mitral regurgitation for which the patient successfully underwent urgent mitral valve replacement. Computed tomography located fragments of the embolised mitral valve leaflet at the bifurcation of the abdominal aorta and within the right common iliac artery. In the absence of impaired lower limb perfusion, the leaflet fragments were left in situ. He was discharged without complication and remains asymptomatic three months post-discharge. Discussion Leaflet fracture and escape is a known but rare complication of mechanical valve prostheses. The first-line investigation for suspected mechanical valve dysfunction is transoesophageal echocardiogram which elicited the diagnosis on this occasion.