To assess the endothelialization of the coils used to close a persistent ductus arteriosus (PDA), the present study comprised a review of the follow-up aortograms and pulmonary angiograms in 25 patients who underwent coil occlusion. The minimal diameter and the length of the PDA were measured prior to the procedure, and the shortest distance between the aortic end of the deployed coil and the aortic end of the PDA was measured after coil deployment. Evidence of endothelial coverage of the coil was sought on follow-up angiograms performed 6-24 (15±5) months later and the factors that determined the thickness of the endothelial coverage on the aortic end were investigated. Separation of the coil and the contrast column were detected at the aortic end in all cases and at the pulmonary end in 18 of 25 cases. The thickness of the separation ranged from 0.4 to 1.3 (0.7±0.2) mm at the aortic end and 0.3 to 0.8 (0.6±0.2) mm at the pulmonary end. The length of the ductus and of the ampulla had a significant positive correlation with the thickness of the aortic end separation. Apparent endothelial coverage of the coil was completed by 6 months after coil occlusion. Infective endocarditis or thromboembolism is an unlikely complication once endothelium covers the implanted coil.