Abstract : One of the most frequent procedures in oral surgery units is the removal of a partially or completely impacted mandibular third molar under local anesthesia and it directly affects the patient's quality of life. The aim of this study is to make a comparison between the effect of the locally applied clindamycin and the systemic clindamycin on occurrence of postoperative alveolar osteitis following surgical removal of impacted mandibular third molars. Fifty patients who had undergone the surgical removal of an impacted mandibular third molar under local anesthesia participated in a randomized prospective clinical study. The patients were distributed in 2 groups. Study group included 25 patients treated with local application of clindamycin inserted inside the socket after surgical removal of impacted tooth, while control group comprised of 25 patients treated with systemic clindamycin one hour before surgical removal of the impacted tooth. The occurrence of alveolar osteitis was evaluated on the 2nd and 7th day postoperatively. Alveolar osteitis occurred on the 48 hours postoperatively. The total incidence of alveolar osteitis was 10%. The incidence of alveolar osteitis was more in control group (16%) when compared to study group (4%) and the difference was statistically significant. Local application of saturated gelfoam by clindamycin reduces the incidence of alveolar osteitis to a significant level when compared to systemic clindamycin.