The purpose of this study was to assess the incidence of flare-ups among patients who received endodontic treatment and to examine the correlation with pre-operative and operative variables.

Analysis was in two aspects (a) overall incidence of flare-ups as expressed by a percentage of all patients visits and (b) percentage of flare-ups that occurred as related to various factors such as patient demographics, diagnosis, and treatment procedures.

1. From the 840 teeth which were examined in this study, the total number of flare-ups was 13.

2. As to gender of patients, there was no significant difference in flare-ups.

3. As to tooth groups, there was no significant difference in flare-ups.

4. In the teeth with pre-operative symptom, there was a statistically significant higher incidence of flare-ups than the teeth without it.

5. In the teeth with apical periodontitis, there was a statistically significant higher incidence of flare-ups.

6. As to pulp and periapical status, non-vital teeth had a higher incidence as compared with vital teeth, irreversible pulpitis.

7. Multi-visit treatment resulted in the higher incidence of flare-ups than one visit treatment.

8. Re-treatment procedures had a statistically significant higher incidence of flare-ups than root canal treatment.

In this study, overall percentages of flare-ups was 1.55%. It showed a statistically significant higher incidence related to pre-operative symptom, apical periodontitis, and re-treatment. There was no significant difference in flare-ups related to gender, tooth groups, and fistula.