The pressure half-time is commonly used to calculate mitral valve area in patients with mitral stenosis, but is inaccurate in many clinical scenarios. Thirteen patients with mitral stenosis underwent routine stress echocardiography with results at rest compared to those at peak exercise. Pressure half-time decreased (-30.6 ± 35.3 ms/l/min) and calculated valve area increased by 0.25 ± 0.30 cm2/l/min with exercise. A previously validated mathematical model of the cardiovascular system yielded similar changes in pressure half-time and calculated valve area. Changes in cardiac output result in predictable changes in pressure half-time and should be considered when performing serial examinations in patients with mitral stenosis.