Background: The Gorlin equation is the reference method for the assessment of aortic valve area in aortic stenosis and is calculated using a constant, called the coefficient of contraction, which is empirically assumed to be 1. This coefficient is the ratio of effective aortic area to anatomic aortic area, and a value of 1 indicates that both are the same. The purpose of this study was to estimate the actual coefficient of contraction in patients with aortic stenosis and to evaluate its impact on aortic area as calculated by the Gorlin equation. Methods: We studied 17 patients with moderate to severe aortic stenosis. Effective aortic area was calculated using the continuity equation. Anatomic aortic area was obtained by planimetry with transesophageal echocardiography. Aortic valve area by the Gorlin equation was calculated from echocardiography data. The coefficient of contraction was derived as above. Results: The coefficient of contraction was inversely related to the pressure recovery. Effective area was correlated with anatomic area (r = 0.86, P < 0.01) but there was a high mean difference (0.22 ± 0.14 cm²). Aortic area by the Gorlin equation was not correlated with anatomic area, but the correlation became significant when the Gorlin equation was corrected for coefficient of contraction and pressure recovery. Conclusions: Using a coefficient of contraction of 1 in the Gorlin equation gives a poor correlation with anatomic area. Using the calculated coefficient of contraction for each patient and the mean gradient for pressure recovery improves the correlation with anatomic area. These facts could be taken in account when Gorlin equation is considered as the reference method.