BACKGROUND About 4% of African Americans possess the isoleucine 122 (V122I) variant of transthyretin, associated with cardiac amyloidosis beyond ages of 55 to 60 years. Transthyretin amyloidosis associated with variant V122I (ATTR V122I) is likely to be an important cause of heart failure in Afro-Caribbean populations, but the high prevalence of left ventricular hypertrophy (LVH) and lack of awareness of this genetic disorder pose diagnostic hurdles. We report the electrocardiographic (ECG) features of ATTR V122I in the largest clinical series to date. METHODS Patients with ATTR V122I were identified in collaboration with the UK National Amyloidosis Centre. The ECG at presentation was assessed for cardiac rhythm, axis, and voltage complex size. RESULTS We include 64 patients with ATTR V122I, with a median age of 74 years (range, 57-88 years). Normal or increased ECG voltage was present in 44.3% of patients, and overall 25% met the criteria for LVH. A significant negative correlation between voltage complex size and duration of illness was seen (P < .05). First-degree heart block was evident in 56% of patients in sinus rhythm. During follow-up (n = 17; median, 28 months), 50% of patients with initial first-degree heart block required pacing. CONCLUSIONS Electrocardiographic voltages meet the criteria for LVH in one quarter of patients with ATTR V122I cardiac amyloidosis. The widely held belief that cardiac amyloidosis is associated with low-voltage complexes is likely to contribute to underdiagnosis of ATTR V122I. First-degree heart block is common at diagnosis and identifies patients at high risk for subsequent pacing requirement.