This paper presents a multi-atlas segmentation approach concerning the left atrium and pulmonary veins in pre-operative CT images in order to plan ablation therapy in patients with atrial fibrillation. The segmentation procedure is composed of an atlas-based segmentation followed by a region-growing method. The atlas-based segmentation step exploits the a priori knowledge of existing structures to extract the inner region of the left atrium. The output of the atlas-based segmentation is then eroded to be used as seed volume in the region-growing procedure. This step adds new voxels according to a criterion which uses the intensity information of the input image.