The purpose of this study was to investigate the clinical variables influencing endodontic flare-ups. Three hundred and thirteen teeth receiving endodontic treatment at the Endodontic Department, Chang Gung Memorial Hospital were studied from December 1992 to February 1993. Among them, 21 teeth with significant pain and 9 with apical swelling were noted after the first appointment of treatment. Three teeth with persistent pain and one with apical swelling were also found one week after completion of endodontic therapy. The results showed significant improvement of clinical symptoms and signs one week after completion of endodontic treatment in comparison with pretreatment and after the first appointment (p < 0.025). The factors such as presence of pretreatment complaints, periapical lesions and vital pulp had significant effects on the incidence of endodontic flare-ups after the first appointment of treatment (P < 0.025). In contrast, patients' sex, the number of visits, and whether this was a retreatment case or not had no significant effect on the frequency of these endodontic flare-ups.