The purpose of this prospective study was to assess the incidence of flare-ups (a severe problem requiring an unscheduled visit and treatment) among patients who received endodontic treatment by the two authors in their respective practices during a period of one year, and also to examine the correlation with pre-operative and operative variables. The results showed an incidence of 1.58% for flare-ups from 1012 endodontically treated teeth. Statistical analysis using the chi-square test (P<0.05) indicated that flare-ups were found to be positively correlated with multiple appointments, retreatment cases, periradicular pain prior to treatment, presence of radiolucent lesions, and patients taking analgesic or anti-inflammatory drugs. In contrast, there was no correlation between flare-up, and age, sex, different arch/tooth groups and the status of the pulp.