We present a case of a 53-year-old woman with intractable shortness of breath that was originally ascribed to bronchiolitis obliterans organizing pneumonia. Subsequently evaluation by echocardiography and cardiac catheterization revealed that she had Lutembacher's syndrome, an uncommon combination of congenital atrial septal defect (ASD) and acquired mitral stenosis that is difficult to diagnose clinically. Our case illustrates the pitfalls and advantages of echocardiographic assessment of the mitral valve area (MVA) and the left atrial pressure (LAP). The pressure half-time method used most commonly for estimating MVA echocardiographically is inaccurate and may lead to underestimation of the severity of mitral stenosis in patients with Lutembacher's syndrome. On the other hand, the presence of ASD provides an additional method of calculating LAP, the most important determinant of symptoms in patients with mitral stenosis.