The Doppler determination of the mitral pressure half-time has gained widespread acceptance as a reliable estimate for mitral valve area, despite little theoretical basis for its "independence" of other hemodynamic variables. A simple model of the left atrium and mitral valve has been developed and a governing equation derived from fluid dynamics fundamentals. Solution of this equation indicates that the pressure half-time should vary inversely with mitral valve area, but also proportionally to net left atrial and ventricular compliance and to the square root of the peak transmitral gradient. This complex relation is apparently masked in the typical clinical situation because pressure and compliance tend to change in opposite directions, thereby partly offsetting each other. In several clinical settings, such as balloon mitral valvotomy, left ventricular hypertrophy and aortic regurgitation, changes in initial pressure and compliance may be large enough to alter the relation between mitral area and pressure half-time. This study reviews the development of the pressure half-time concept, presents an overall method for studying mitral valve flow using mathematical modeling and describes the effects of factors other than mitral valve area on pressure half-time.