Abstract Background Conventional complete left bundle branch block (CLBBB) criteria sometimes result in a false‐positive diagnosis that does not represent dyssynchrony. Recently, true CLBBB criteria have been proposed to detect responders to cardiac resynchronization therapy (CRT), although their correlation with severity of dyssynchrony or natural prognosis is unclear. Methods Ninety‐four consecutive patients (74 ± 9 years, 63 men) with conventional CLBBB during sinus rhythm underwent semiconductor SPECT. They were divided into two groups: patients with true CLBBB and others. True CLBBB was characterized by the mid‐QRS notching/slurring and wide QRS duration (male, ≥140 milliseconds; female, ≥130 milliseconds). Multivariate analysis was performed to detect left ventricular dyssynchrony (LVD), defined as bandwidth ≥145° and/or phase standard deviation (SD) ≥43°. Primary endpoints (hospitalization for heart failure or cardiac death) were evaluated. Results True CLBBB had wider bandwidth (145 ± 83° vs 110 ± 64°, P = 0.024) and higher phase SD (48 ± 26° vs 35 ± 19°, P = 0.007). Ejection fraction (EF), end‐diastolic volume (EDV), summed rest score (SRS), and the presence of ischemic heart disease (IHD) showed no differences between groups (P = 0.401, 0.591, 0.165, and 0.212, respectively). Multivariate analysis revealed that true CLBBB, EF, and EDV were significant predictors of LVD (odds ratio, 12.6, 0.90, 1.03; P = 0.003, 0.002, 0.022, respectively). At 3‐year follow‐up (median 667 days), primary endpoints were comparable in both groups (log‐rank, P = 0.92). Conclusions Patients with true CLBBB had more severe dyssynchrony on single‐photon emission computed tomography than patients with nontrue CLBBB. On the other hand, the two groups showed no differences in EF, EDV, the presence of IHD, hospitalization for heart failure, and cardiac death.