Objectives: This study was aimed at evaluating the usefulness of giant negative T-wave (GNT) as an index of apical hypertrophic cardiomyopathy (ACM) severity and as a better echocardiographic index to represent apical hypertrophy. Methods: Seventy-five patients who were recently diagnosed with ACM by echocardiography were enrolled in this study. ACM patients were divided into two groups: group 1 (ACM with GNT) and group 2 (ACM without GNT). To evaluate ACM severity, apical wall thickness (A-WT) and apical cross-sectional muscle area (A-CSMA) were measured by echocardiography. Results:Twenty-seven patients (36%)hadGNT.ThemaximalA-WT ofgroups1and2was 19.7 ±2.4 and 18.6 ± 2.0 mm, P = 0.027, respectively. In addition, the maximal A-CSMA differed significantly between groups 1 and 2 (14.2 ± 1.8 vs. 11.8 ± 1.9 cm 2 ,P < 0.001). In the correlation analysis, Tmax showed a stronger correlation with A-CSMA than with A-WT (r = 0.599 vs. r = 0.291). Conclusions: These results indicate that the presence of GNT in ACM patients may represent more severe apical hypertrophy. Furthermore, A-CSMA may be a more reliable ACM severity index than A-WT. (Echocardiography 2010;27:770-776)