In the case of a failed endodontic treatment, it is necessary to re-treat the root canal. Orthograde endodontic retreatment involves removing the existing filling material from the root canal with additional cleaning, disinfection and final obturation of the canal. An important step during retreatment is to completely remove the existing filling material in order to enable access to all parts of the root canal system. However, previous studies report about the inability to completely remove obturation material from the root canal walls, regardless of the techniques or instruments used for retreatment, or the type of materials removed. The aim of a study, conducted on 120 extracted teeth was to examine the cleanliness of root canal walls after retreatment. After root canal preparation the sample was divided into two groups and obturated using two different materials (gutta-percha and resilon). Depending on the technique of removing these materials, the groups were further divided in relation to the used instruments (Hedstrom, ProTaper and Twisted File) and in relation to the use of solvents (with and without chloroform). After completing the retreatment, the roots were observed on SEM after splitting them longitudinally. Photomicrographs of each third of the canal were made at 500x, and the amount of remaining filling material was evaluated by using a grading scale. Analysis of the results indicated a higher cleanliness of canal walls after the removal of gutta-percha than after resilon removal. Rotary instruments were more efficient while removing gutta-percha, and manual Hedstrom files efficiently removed resilon. The use of solvent did not significantly affect the amount of residual material on the canal walls. Reduced cleanliness after resilon removal probably is the result of adhesive bonding of this material to the canal walls, and formation of a "monoblock". It is believed that a combination of manual and mechanical methods could result in better quality of canal walls cleaning.