The aim of the present study is to compare the antimicrobial and immediate clinical effect of NaOCl 0.5% with the effect of NaOCl 3% in the endodontic treatment of teeth referred to the Specialist Clinic of Endodontics, Göteborg, Public dental Health, Västra Götaland. The efficiency of the treatment will be evaluated with sampling and cultivation methods and the assessment of patients' symptoms with a visual analogue scale.