The diagnostic evaluation of patients with isolated left bundle branch block (LBBB) is challenging due to limitations of several non-invasive tests. Our aim was to evaluate the diagnostic value of cardiovascular magnetic resonance (CMR) in asymptomatic patients with LBBB. Sixty-one asymptomatic patients with complete LBBB who were referred for CMR from January 2005 to November 2010 were identified. 29 patients (18 men) had normal echocardiograms (echo) whereas 25 (18 men) had abnormal findings on echo. Six had no echo and one had poor echo windows, and these patients were excluded from further analysis. Patients with cardiac symptoms or known coronary artery disease at the time of referral were also excluded. Of the 29 patients with normal echo, 9 (31%) were found to have pathological findings on CMR. The most common abnormalities were dilated cardiomyopathy-DCM (n = 6, 21%) followed by left ventricular hypertrophy (n = 2, 7%). Of the 25 patients who had an abnormal echo, CMR confirmed the diagnosis in 19 (76%) and provided clinically relevant additional information in 13 (52%) subjects. Of these 13 patients, 9 (69%) had characteristic patterns of myocardial late gadolinium enhancement (8 mid-wall and 1 patchy distribution consistent with DCM and cardiac sarcoid, respectively). CMR detects sub-clinical cardiomyopathy in a third of asymptomatic patients with LBBB despite normal echocardiograms. In those with abnormal echocardiograms, CMR provides additional clinically relevant information in over 50% of patients, including a high prevalence of mid-wall fibrosis in patients with impaired left ventricular function. These findings support the use of CMR as a valuable adjunct to conventional investigations in asymptomatic LBBB.