In this centry, much work has been undertaken to clarify the underlying pathophyoiological proceses of acute sinusitis. In previous studies, ovstruction of natural ostium and pathogenic bacteria seems to be of crucial importance in the deveopment of sinusitis. But the pathophysiology of acute sinisitis is still not competely known. In order to invetgate the histopathological mechanism of acute sinusitis, we study the bacteriologic and pathologic features in the experimentally induced acute sinusitis in rabbits. We occluded the natural ostium by butylcyanocrylte and inoculated 106 Bacteroides fragilis for experimentally induced anareobic sinusitis. The animals were divided into 4 groups ; Group A : injection of Bacteroides fragilis after blocking of natural ostium. Group B : injection of Bacteroides fragilis without blocking of natural ostium. Group C : blocking of natural ostium without injection of Bacteroides fragilis. Group D : dropping of butylcyanocrylate at sinus mucosa without any other procedure. In this study, the results were as follows ; 1) On the 5th postoperative day, acute maxillary sinusitis was developed in all cases of Group A, in some cses of Group B and C. In Group C, more cases were developed acute maxillary sinusitis than Group B. But in Group D, no case was developed acute sinusitis. 2) The inoculated Bacteroides fragilis was reisolated in Group A and Group B. But in Group A, more cases were reisolated. In bacteriological study, the isolated bacteria in postoperative rabbits was similar to peroperative rabbits exceppt pneumococcus. 3) In microscopic examination on the 5th postoperative day, the sinus mucosa of Group C showed regeneration of clila that represented as healing process. But Group A not reveled the hearing process, and demonstrated a case of empyema formation. 4) In pathological study, the sinus mucosa revealed damage of ciliated epithelium and inflammatory cell infiltration within postoperative 24 hours. But the bacteria was not demonstrated in sinus mucosa throughout entire experimental periods. In conclusion, the Bacteroides fragilis had a high virulence and the ability to resist the normal degense mechanism of the sinus mucosa. Obstruction of natural ostium is plays a primary etiologic role in development of sinusitis.