Purpose: To determine whether neglecting the flow unsteadiness in simplified Bernoulli's equation significantly affects the pulmonary transvalvular pressure drop estimation. Materials and Methods: 3.0T magnetic resonance imaging (MRI) 4D velocity mapping was performed on four healthy volunteers, seven patients with repaired tetralogy of Fallot, and thirteen patients with transposition of the great arteries repaired by arterial switch. Pulmonary transvalvular pressure drop was estimated based on two methods: General Ber-noulli's Equation (GBE), ie, the most complete form; and Simplified Bernoulli's Equation (SBE), known as 4V 2. More than 2300 individual pressure drop measurements were used to compare the simplified and the general Bernoulli's methods. A linear mixed-effects model was employed for statistical analyses, fully accounting for clustering of observations among the methods and systolic phases. Results: The simplified Bernoulli's method systematically underestimated the pressure drop compared to general Bernoul-li's method during the entire systolic phase (P < 0.05), including the peak systole, where on average Dp SBE =Dp GBE 578%. Conclusion: The simplified Bernoulli method underestimated the pressure drop during all systolic phases in all the studied subjects. Therefore, it is necessary to take into account the flow unsteadiness for more accurate estimation of the pressure drop. J. MAGN. RESON. IMAGING 2015;00:000-000. P ulmonary valve stenosis is a common anomaly in patients with congenital heart defects. Quantitative assessment of the severity of pulmonary valve stenosis is mainly based on the pulmonary transvalvular pressure drop, 1 commonly estimated by continuous-wave Doppler 2 and recently by magnetic resonance imaging (MRI) phase contrast flow measurements. 3 The accuracy of noninvasive measurements of the transvalvu-lar pressure drop has been debated. 4-11 Several studies have investigated the correlation between MRI measurements and echocardiography with catheterization for the estimation of the pressure drop in pulmonary circulation. The results have been inconsistent. 3,12-14 Cardiac catheterization is the current standard for estimating the transvalvular pressure drop; however , this method is not practical for routine follow-ups due to View this article online at wileyonlinelibrary.com.