In hypertrophic cardiomyopathy (HC), electrocardiographic (ECG) changes have been postulated to be an early marker of disease, detectable in sarcomere mutation carriers when left ventricular (LV) wall thickness is still normal. However, the ECG features of mutation carriers have not been fully characterized. Therefore, we systematically analyzed ECGs in a genotyped HC population to characterize ECG findings in mutation carriers (G+) with and without echocardiographic LV hypertrophy (LVH), and to evaluate the accuracy of ECG findings to differentiate at-risk mutation carriers from genetically unaffected relatives during family screening. The ECG and echocardiographic findings were analyzed from 213 genotyped subjects (76 G+/LVH-, 57 G+/LVH+ overt HC, 80 genetically unaffected controls). Cardiac magnetic resonance imaging was available on a subset. Q waves and repolarization abnormalities (QST) were highly specific (98% specificity) markers for LVH- mutation carriers, present in 25% of G+/LVH- subjects, and 3% of controls (p