The aim of this study was to assess the therapeutic effects of antibiotic delivered by polyactic acid (PLA) polymer in sinusitis. Maxillary sinusitis was induced in 27 New Zealand white rabbits by occluding the sinus ostium and inoculating the sinus cavity with Streptococcus pneumoniae serotype 3. The rabbits were divided into 3 groups consisting of group 1 (control group, 9 rabbits) that was treated only by reopening the ostium only, group 2 that was treated by both reopening the ostium and injecting ampicillin intramuscularly (9 rabbits), and group 3 (9 rabbits) in which a piece of PLA-polymer ampicillin sheet was placed within the sinus after reopening the ostium. The histologic findings including epithelial ulceration, loss of cilia, infiltration of inflammatory cells, and edema were mild to moderate in group 2 and minimal in group 3. The ultrastructual findings including swelling of mitochondria and endoplasmic reticulum, and protruded cytoplasm were severest in the control group, followed by groups 2 and 3. The mucociliary transport speed measured at the medial wall of the maxillry sinus was highest in group 3. The results of this study suggest that treatment with PLA-polymer ampicillin may have better therapeutic effects in maxillary sinusitis than that with systemic administration of ampicillin.