Methodology: Consecutive patients referred to a specialist clinic of endodontics are randomly assigned to have the root canal treatment performed with a concomitant irrigation either with a buffered 0.5 % NaOCl (Dakin's solution) or 3 % NaOCl. Teeth with vital or necrotic pulps and retreatment cases were included. Root canal treatment procedures varied but followed the routines of the Specialist Clinic of Endodontics, Public Dental Health, Gothenburg, Sweden. Immediately before the root canal filling a microbial sample is taken from the operative field and the root canal. The bacterial samples were processed at the laboratory as outlined by Möller (1966). After each visit each patient was instructed to fill in a questionnaire and assess his or her post-operative pain on a visual analogue scale (VAS), with endpoints 0= "no pain" and 10= "worst imaginable pain", for seven consecutive days. The Fischer's exact test was used for statistical analyses of the differences in outcome between the groups.