Objective: Longer QRS duration on the ECG is associated with increased cardiovascular (CV) mortality. To evaluate potential mechanisms, we examined in this study the relationship between QRS duration and left ventricular (LV) mass and LV end systolic and end diastolic volume in patients with known CV disease or high-risk diabetes. Methods: In a substudy of the ONTARGET/TRANSCEND (Ongoing Telmisartan Alone and in Combination with Ramipril Global Endpoint Trial/Telmisartan Randomised Assessment Study in ACE Intolerant Subjects with Cardiovascular Disease) clinical trials, 368 patients had a cardiac magnetic resonance scan to measure LV mass, LV end systolic volume, LV end diastolic volume and LV ejection fraction at baseline and after 2 years of followup. Relationships between QRS duration on the 12-lead ECG and LV mass and volumes were evaluated at both assessments. Results: Each 10-ms increase in QRS duration both within and above the normal reference range was associated with an 8.3% (95% CI 6.7% to 9.9%) increase in LV mass, a 9.2% (95% CI 7.4% to 10%) increase in LV end diastolic volume and a 7.8% (95% CI 6.4% to 9.3%) increase in LV end systolic volume. QRS duration increased with body size, but associations with LV mass and volumes remained strong after indexing measurements to height 2.7 (p<0.001 for all) and were similar for subjects with an otherwise normal and abnormal ECG. Conclusion: A longer QRS duration both within and above the normal reference range is associated with a greater LV mass and larger LV end systolic and end diastolic volumes. This may explain the known association of longer QRS duration with increased CV mortality.