Background-We sought to compare maximal left ventricular (LV) wall thickness (WT) measurements as obtained by routine clinical practice between echocardiography and cardiac magnetic resonance (CMR) and document causes of discrepancy. Methods and Results-One-hundred and ninety-five patients with hypertrophic cardiomyopathy (median age, 52.8±15.1 years) who underwent echocardiography and CMR imaging within 6 months (median, 41 days; interquartile range, 16-97 days) were included. LVWT was assessed in parasternal long and short axis by 2-dimensional echocardiography and in short axis by CMR. By Bland-Altman plot, mean maximal LVWT difference between echocardiography and CMR was 0.5 mm (95% confidence interval, −6.9, 7.8) with equal distribution of discrepancy along the full range of LVWT. Ninety-seven patients (49.7%) were identified to have intermodal measurement discrepancies ≥10%. In 7 patients (7.2%), reported measurement by CMR was inaccurate because of interpretation error. In 90 patients (92.8%), echocardiography underestimated (n=32; 33.0%) or overestimated (n=58; 59.8%) maximal LVWT. Underestimation was because of focal LV hypertrophy (n=10; 10.3%) or poor acoustic windows (n=22; 22.7%) while overestimation resulted from inclusion of right ventricular myocardium (n=37; 38.1%), LV trabeculations (n=5; 5.2%), papillary muscle (n=3; 3.1%), and apical-septal bundle (n=1; 1.0%), as well as imaging plane obliquity (n=7; 12.5%). In 31 (15.9%) patients, measurement discrepancy occurred at diagnostic or prognostic cut-offs. Conclusions-Although maximal LVWT by echocardiography in general measured similar to CMR, discordance because of limitations in echocardiography technique was present in a significant subset of patients. As measurement of LVWT impacts diagnosis and sudden death management, CMR should be considered as part of routine evaluation of all patients with hypertrophic cardiomyopathy. (Circ Cardiovasc Imaging. 2017;10:e006309.