['Up to one-third of the patients who undergo cardiac resynchronization therapy (CRT) are not responders.', 'To demonstrate that delayed lateral left ventricular activation time determined through time to intrinsicoid deflection onset (ID) predicts response after CRT.', 'The ID in leads I, aVL, V₁ and V₂, and V₅ and V₆ were measured in 135 patients who underwent CRT. A CRT response was defined as a decrease in left ventricular end-systolic volume (LVESV) exceeding 15% at 6 months.', 'In patients with left bundle branch block or nonspecific intraventricular conduction delay, response was predicted by longer ID in lead I (odds ratio [OR] 3.23; 95% confidence interval (CI) 1.4-7.4; per 20-ms increase), in lead aVL (OR 3.0; 95% CI 1.2-7.3; per 20-ms increase), and in lead I minus lead V₁ (OR 2.4; 95% CI 1.2-4.7) adjusting for baseline QRS duration and LVESV. Results were similar after adjusting for postimplant or change in QRS duration. The ID parameters were better predictors of response than QRS duration parameters. ID in lead I/QRS duration ratio (OR 3.1; 95% CI 1.6-5.9) also increased the odds of response after adjusting for baseline LVESV. Cutoff values for ID in leads--I, 110 ms; aVL, 130 ms; I minus V₁, 90 ms--and ID in lead I/QRS duration ratio of 0.69 yielded a sensitivity and a specificity as high as 83% and 81%.'] Does delayed intrinsicoid deflection onset in surface ECG lateral lead predicts left ventricular reverse remodeling after cardiac resynchronization therapy?