Abstract Funding Acknowledgements Type of funding sources: None. Background Despite the increasing number of studies concerning Left Atrial Strain (LAS), few data are available comparing LAS patients with cardiac amyloidosis (CA) and sarcomeric hypertrophic cardiomyopathies (HCM). Purposes We aimed to perform a comparative multimodal imaging analysis of LAS of a prospective cohort of patients with CA and HCM. Methods For each enrolled patient, we performed same-day two and three-dimension echocardiography (TTE) and cardiac magnetic resonance imaging (CMR) to blindly measure the peak atrial longitudinal strain (PALS) and the peak atrial contraction strain (PACS). Patients with acute atrial fibrillation were excluded. Results Between January 2020 and July 2021, 67 patients were included: 31 patients with CA (age 75.1 ± 10 years, left ventricular ejection fraction 60.6 ± 10.4%, maximum left ventricular thickness 17.8 ± 3.9 mm) and 36 with HCM (age 50.8 ± 15.5 years, left ventricular ejection fraction 66.1 ± 9.8%, maximum left ventricular thickness 20.7 ± 4.5 mm). Left atrial volume was similar in the 2 groups (42.5 ± 15.6 mL/m2 in HCM vs 47.9 ± 15 in CA, P = 0.1557). Concerning PALS, its values for CA and for CMH were on 2D TTE manual (10.9 ± 5.8% vs 21.4 ± 9.4%, P < 0.001), 2D TTE automatic (11.5 ± 7.3% vs 22.9 ± 10.2%, P < 0.001), 3D TTE (10 ± 6.8% vs 18.1 ± 6.7%, P < 0.001), and CMR (11.3 ± 8 vs 24.4 ± 17.1, P < 0.001) respectively. Concerning PACS, its values for CA and for CMH were on 2D TTE manual (5.2 ± 3.4% vs 10 ± 4%, P < 0.001), 2D TTE automatic (4.9 ± 3.9% vs 10.2 ± 5.1%, P < 0.001), 3D TTE (3.6 ± 3.8% vs 7.9 ± 4%, P = 0.001) and CMR (6.2 ± 5.8% vs 11.9 ± 9%, P = 0.004) respectively. Multivariate analysis adjusted on main factors influencing LAS (left ventricular (LV) mass, LV ejection fraction, LV global longitudinal strain, renal function and history of hypertension) found that the differences between the two groups remained significant for PALS and PACS for almost all technics. Furthermore, although concordance between the 3 echocardiographic technics was excellent (the interclass correlation coefficient (ICC) was higher than 0.80 between each TTE methods), ICC was poor between TTE and CMR technics: 0.40 (0.18-0.59) for manual 2D TTE and CMR, 0.46 (0.24-0.63) for auto 2D TTE and CMR, 0.40 (0.14-0.60) for 3D TTE and CMR. Conclusion Our study is the first to describe and compare, both on TTE and CMR, LAS on a prospective cohort of patients with CA and HCM. Although they have same mean left atrial volume, we found significant differences on PALS and PACS between these two groups of patients on all the studied technics. These findings may be used in future multi-modality imaging studies dealing with diagnosis or prognosis of these hypertrophic cardiopathies. Abstract Figure. LAS 2D TTE manual Abstract Figure. LAS comparison between CA and HCM