Objective To evaluate left ventricular (LV) function and dyssynchrony in patients with dilated cardiomyopathy (DCM) and complete left bundle branch block (CLBBB) by three-dimensional speckle tracking imaging(3D STI).Methods 3D STI was performed and analyzed using TomTec 4-D LV analysis 3.0 software in 37 DCM patients with CLBBB and 25 healthy volunteers.The global 3D,longitudinal,circumferential,radial strains were measured.LV dyssynchrony was evaluated by the standard deviation of time to peak from 3D strain of 16 segments related to the heart cycle(3D-SDI).Results In control group,uniformity in the average value of 3D strain was observed between apical,mid-ventricular and basal levels (P > 0.05).Global 3D,longitudinal,radial and circumferential strains had excellent correlations with LV ejection fraction ( r =- 0.92,- 0.84,- 0.78 and 0.81,respectively,P <0.01).Compared with control group,global 3D,longitudinal,radial and circumferential strains were significantly lower in DCM patients ( P <0.01 for all).3D-SDI in DCM patients with CLBBB was significantly longer than that of volunteers ( P <0.01).3D-SDI increased with worsening LV systolic function regardless of QRS duration (P <0.05).Conclusions When image quality is optimal,3D STI represents a promising novel technique for assessment of global LV function and dyssynchrony. Key words: Echocardiography; Cardiomyopathy, dilated; Ventricular function, left; Speckle tracking imaging