Mitral stenosis is a progressive lesion carrying a relatively high risk of sudden incapacitation from systemic embolus or the onset of atrial fibrillation. Since the condition is likely to be significant when diagnosed, it is not compatible with single-crew professional operations and requires careful supervision. The presence of mild mitral regurgitation where the aetiology has been shown not to be due to chordal rupture, papillary muscle dysfunction secondary to coronary artery disease, rheumatic mitral valve disease, or Marfan's disease, where left atrial and left ventricular dimensions are shown to be normal on the echocardiogram, and where follow-up over at least a year has shown no progression of disease, may be consistent with full certification. Regular cardiological review with echocardiography and exercise electrocardiography is required. Any departure from these guidelines may lead to restriction of flying status to multi-crew operations, or denial.