Background: We examined the agreement between transthoracic echocardiography (TTE) and intraoperative prepump transesophageal echocardiography (TEE) in the assessment of left-sided regurgitant lesions and echocardiographic variables associated with grading discrepancies. Methods: The TTE and prepump TEE studies of 54 patients undergoing aortic-valve replacement for aortic stenosis were reviewed. Agreement and correlation in assessment of aortic (AR) and mitral regurgitation (MR) severity were evaluated. Results: There was no significant difference between mean TTE and prepump TEE grading of MR (0.23 ± 0.19 vs. 0.21 ± 0.15 jet area/area of the left atrium, p = 0.49), but the correlation between the two methods was weak (r = 0.40, p = 0.003), with an exact agreement of 54%. Prepump TEE tended to grade AR as more severe (mean grade 1.43 ± 0.94 vs. 1.24 ± 0.75, p = 0.058). The correlation between the two methods in AR assessment was fair (r = 0.70, p = 0.0001) with an agreement of 59%. For MR and AR grading, no significant correlations between valvular regurgitation severity and blood pressure differences between preoperative TTE and prepump TEE were found. In 17% of cases, discrepancies in identifying severe mitral or aortic valve regurgitation could have affected patient management. Conclusions: There is modest agreement in MR and AR assessment between TTE and prepump TEE. Cardiologists, cardiac surgeons, and anesthesiologists must be aware of differences between these methods when using prepump TEE to guide intraoperative decisions.