Endodontic treatment aims to eliminate pathogenic microbes from the root canal. This is an almost impossible task as microbes, especially Enterococci are able to line the dentin tubules that are nearly unaccessable to desifectants used during treatment. Hyperpure chlorine dioxide is an endodontic irrigant. In vitro studies proved it to be a very potent desinfectant against which bacteria do not produce resistency. It selectively binds to three amino acids targeting bacterial cells. Due to its small molecular weight and its highly volatile characteristics we presume that it is able to penetrate into the small dentin tubuli. There have been many in vitro studies comparing its characteristics and efficacy, but there have been no in vivo studies done to approve its clinical preformance so far. Our goal is to compare the efficacy of hyperpure chlorine dioxide with the gold standard sodium hypochlorite in the non-surgical retreatment of root canal treated teeth with chronic periapical lesions in a randomized clinical trial. Its efficacy will be compared by microbiological methods and by clinical and radiological evaluation. According to our hypothesis hyperpure chlorine dioxide is more effective antimicrobial agent than the gold standard sodium hypochlorite. The protocol for this clinical study was approved by the Hungarian National Ethics Committee for clinical trials. Forty patients having chronic apical periodontitis will randomly be chosen for the study from the patients attending our Department of Conservative Dentistry. Written consent will be collected from the patients. Only those patients will be accepted who had the root canal treatment done more than four years ago, the tooth is asymptomatic, functionally and esthetically restorable, the coronal restoration can be done within 3-4 weeks of the root canal obturation, who will not have prosthetic or orthodontic treatment interfering with the radiological follow-up and have no general medical conditions or medication influencing the healing of the periapical lesion. Endodontic treatment will be done according to professional quidelines. After sterilizing the tooth and its environment, a sterility test will be be done by taking samples from the top of the restoration (S1) and from the pulp chamber (S2). The root canal obturation will be removed mechanically and a microbiological sample will be taken from the root canal (M1). After taking the sample from the root canal the tooth will be chemomechanically prepared. The patients will randomly be divided into two groups. In the first (control) group sodium hypochlorite, in the second group hyperpure chlorine dioxide will be used as desinfectant. The sample taking procedure will be repeated after one week of temporization (sterility tests S3, S4 and root canal sample M2). The samples will be cultured and identified by MALDI-ToF MS. The remaining samples will be used for PCR testing (Micro-IDent Plus11, Hain Lifescience) to determine the presence of the tested bacteria. In the second session the root canal obturation will be completed. Following coronal restoration a control x-ray will be done by using an individual bite-block for standardization. The success of the root canal treatment will be followed-up by evaluating the periapical status with the Periapical Index scoring system and clinical evaluation according to the criteria set by the European Society of Endodontology including pain assessment where the patients rate pain on the VNRS, with 0 = ''no pain'' to 10 = ''worst pain imaginable". Clinical evaluation and control x-rays will be done at 6 months, 1, 2, 3, and 4 years following treatment. The x-rays will be evaluated by two blinded calibrated examiners.