Obstruction of the natural ostium is known to be one of the major factors in the pathogenesis of maxillary sinusitis. To explore the therapeutic effect of ostial patency, sinusitis was induced in 32 rabbits by obstruction of the natural ostium and inocculation of Streptococcus pneumoniae. They were divided into two groups: the first ("open") group included 16 rabbits with experimentally induced sinusitis, from which bone chips for obstruction of the natural ostium were removed after a period of 5 days, and the second ("closed") group included 16 rabbits with continuous obstruction of the natural ostium. The induction of sinusitis was found to be successful in all cases, when examined after five days. The amount of nasal and sinus secretion as well as histopathological findings of the sinus mucosa were investigated with light microscopy, scanning and transmission electron microscopy, two and four weeks after induction of sinusitis. The findings were significantly improved in the "open" group.