Aims: This study was aimed to evaluate the effect of azithromycin in reducing pathological gingival enlargement. Materials and methods: twelve adult New Zealand male rabbits weighing (900gm-1250mg) and almost the same in age and circumstance were chosen in this study. All animals with pathological gingival enlargement were clinically examined by using a periodontal probe and measuring their gingival sulcus depth. Then, six of them were slaughtered and considered the control group and the other six were experimental group, and at the end of study a specimens of oral mucosa were taken for immunohistochemical examinations. Results: Clinical examination shows there is no reduction in gingival enlargement in the control group while this enlargement decreased in the experimental group after use of azithromycin. Immunohistochemical analysis shows mild matrix metalloproteinase-1(MMP1) expression in the control group and moderate to strong MMP1 expression in sub-epithelial of gingiva in the experimental group. Conclusion: This study establishes that azithromycin is clinically and immunohistochemically effective in the reduction of pathological gingival enlargement.