Objectives This study evaluated the effect of aortic insufficiency on the correlation of pressure half-time–derived mitral valve area with each of 2 standards for mitral valve area (planimetry and cardiac catheterization) in a prospectively assembled cohort of patients scheduled for percutaneous balloon mitral commissurotomy. Background Although Doppler pressure half-time has been validated as a method for assessing mitral valve area, most previous studies have suggested that this noninvasive technique overestimates mitral valve area in the setting of coexistent aortic insufficiency. Methods and Results Echocardiography and cardiac catheterization were performed on 212 consecutive patients scheduled for percutaneous balloon mitral commissurotomy. After excluding 35 patients who did not have aortography, the rest were divided into a “no aortic insufficiency [AI] group” (n = 146) including those with trivial or no aortic insufficiency at catheterization and an “AI group” (n = 31) including those with mild or moderate aortic insufficiency. The pressure half-time mitral valve area tended to slightly underestimate invasive valve area by 0.04 cm² in the AI group and to slightly overestimate invasive valve area by 0.06 cm² in the no AI group. This difference between the groups was not statistically significant (P = .13). The pressure half-time mitral valve area tended to underestimate planimetered valve area by 0.11 cm² in the AI group and by 0.10 cm² in the no AI group. There was no difference between the 2 groups (P = .94). Potential confounders that could theoretically mask the effect of aortic insufficiency on the pressure half-time (including age, heart rate, blood pressure, left ventricular diastolic pressure, ejection fraction, mitral regurgitation, and atrial fibrillation) were excluded by multivariable analyses. Conclusions The pressure half-time method of determining mitral valve area is not adversely affected by mild to moderate aortic insufficiency. This finding has implications for the utility of this technique in the rheumatic valvular disease population, in which mitral and aortic valve disease frequently coexist. (Am Heart J 1998;136:718-23.)