The purpose of this clinical study is to assess whether calcium hydroxide as an intracanal medication affects post-treatment pain in teeth especially odontogenic pain which comes from inflammation of the pulp and periradicular tissues when compared with no intracanal medication. From 213 patients who has been treated 237 root canals due to significant pain (moderate-to-severe) we recorded their age, sex, treated tooth, degree of pain, pre-operative states of the tooth. We classified patients into 2 test group; Group 1 (not gain intracanal $Ca(OH)_2$), Group 2 (gain intracanal $Ca(OH)_2$). Through the survey from the patients, we let them write down the occurrence and degree of post-treatment pain in 4hours, 2days, 7days after treatment as none, mild, moderate or severe. The followings were evaluated ; the overall incidence of flare-ups, the overall incidence of post-treatment pain in each group at each time period , the incidence of post-treatment pain in each group at each time period as related to pre-operative states of the teeth These were compared statistically with Chi-square analysis (p < 0.05). Under the condition of this investigation, no difference was observed in the incidence of post-treat-ment pain between the two groups. Therefore, $Ca(OH)_2$ as intracanal medication had no effect on preventing or decreasing the post-treatment pain.